Remove redundant trampoline PGD for 64bit and add more comment
for why 32bit systems need trampoline PGD.
There was a patch and discussion similar to this,refer to
the link [1][2].
The trampoline PGD is redundant for 64bit systems because:
1. The early PGD covers the entire kernel mapping. Directly
loading early PGD can achieve the result in boot stage.
A more trampoline PGD makes code hard to understand.
2. Directly loading early PGD is safe in 64bit systems since
the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx,
which has a very big gap with RAM address.It won't fall into
the corner case that 32bit system worrys.
3. Remove redundant trampoline PGD can benefit to code maintaince,
because 64bit systems have more page table levels.For example:
If we want to support SV48 which has 4 page table levels, we have
to add a trampoline_pud and insert it before trampoline_pmd.
Reference link:
[1]https://lore.kernel.org/linux-riscv/[email protected]/
[2]https://lkml.org/lkml/2019/3/28/147
Signed-off-by: Nanyong Sun <[email protected]>
Reviewed-by: Alexandre Ghiti <[email protected]>
Reviewed-by: Kefeng Wang <[email protected]>
Reviewed-by: weiyongjun <[email protected]>
---
v2 changes:
Adjust codes based on the review suggestions from Alex Ghiti,
make codes more readable.
---
arch/riscv/kernel/head.S | 46 ++++++++++++++++++++++++++--------------
arch/riscv/mm/init.c | 16 ++++----------
2 files changed, 34 insertions(+), 28 deletions(-)
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index fce5184b22c3..3816aa5edc69 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -89,29 +89,52 @@ relocate:
add ra, ra, a1
/* Point stvec to virtual address of intruction after satp write */
- la a2, 1f
+#ifdef CONFIG_64BIT
+ la a2, load_done
+#else
+ la a2, load_kernel_pgd
+#endif
add a2, a2, a1
csrw CSR_TVEC, a2
- /* Compute satp for kernel page tables, but don't load it yet */
+ /* Compute satp for kernel page tables.
+ * For 64bit systems, load it and trap to stvec.
+ * For 32bit systems, don't load it yet.
+ */
srl a2, a0, PAGE_SHIFT
li a1, SATP_MODE
or a2, a2, a1
/*
+ * Before writing satp, we need a full fence here because setup_vm() just
+ * wrote these PTEs and we need to ensure the new translations are in use.
+ */
+ sfence.vma
+#ifndef CONFIG_64BIT
+ /*
+ * 32bit systems need firstly loading a trampoline to handle a corner
+ * case where load address range overlaps kernel virtual address range.
* Load trampoline page directory, which will cause us to trap to
- * stvec if VA != PA, or simply fall through if VA == PA. We need a
- * full fence here because setup_vm() just wrote these PTEs and we need
- * to ensure the new translations are in use.
+ * stvec if VA != PA, or simply fall through if VA == PA.
*/
la a0, trampoline_pg_dir
XIP_FIXUP_OFFSET a0
srl a0, a0, PAGE_SHIFT
or a0, a0, a1
- sfence.vma
csrw CSR_SATP, a0
+#endif
.align 2
-1:
+load_kernel_pgd:
+ /*
+ * Switch to kernel page tables. A full fence is necessary in order to
+ * avoid using the trampoline translations, which are only correct for
+ * the first superpage. Fetching the fence is guarnteed to work
+ * because that first superpage is translated the same way.
+ */
+ csrw CSR_SATP, a2
+ sfence.vma
+
+load_done:
/* Set trap vector to spin forever to help debug */
la a0, .Lsecondary_park
csrw CSR_TVEC, a0
@@ -122,15 +145,6 @@ relocate:
la gp, __global_pointer$
.option pop
- /*
- * Switch to kernel page tables. A full fence is necessary in order to
- * avoid using the trampoline translations, which are only correct for
- * the first superpage. Fetching the fence is guarnteed to work
- * because that first superpage is translated the same way.
- */
- csrw CSR_SATP, a2
- sfence.vma
-
ret
#endif /* CONFIG_MMU */
#ifdef CONFIG_SMP
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index ac48742fa6fc..306fcb2334fa 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
EXPORT_SYMBOL(pfn_base);
pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
+#ifndef CONFIG_64BIT
pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
+#endif /* CONFIG_64BIT */
static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
#ifdef CONFIG_XIP_KERNEL
+#ifndef CONFIG_64BIT
#define trampoline_pg_dir ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
+#endif /* CONFIG_64BIT */
#define fixmap_pte ((pte_t *)XIP_FIXUP(fixmap_pte))
#define early_pg_dir ((pgd_t *)XIP_FIXUP(early_pg_dir))
#endif /* CONFIG_XIP_KERNEL */
@@ -300,13 +304,11 @@ static void __init create_pte_mapping(pte_t *ptep,
#ifndef __PAGETABLE_PMD_FOLDED
-static pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
static pmd_t early_dtb_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
#ifdef CONFIG_XIP_KERNEL
-#define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
#define fixmap_pmd ((pmd_t *)XIP_FIXUP(fixmap_pmd))
#define early_pmd ((pmd_t *)XIP_FIXUP(early_pmd))
#endif /* CONFIG_XIP_KERNEL */
@@ -585,16 +587,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
/* Setup fixmap PMD */
create_pmd_mapping(fixmap_pmd, FIXADDR_START,
(uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
- /* Setup trampoline PGD and PMD */
- create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
- (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
-#ifdef CONFIG_XIP_KERNEL
- create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
- kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
-#else
- create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
- kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
-#endif
#else
/* Setup trampoline PGD */
create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
--
2.18.0.huawei.25
Le 28/07/2021 ? 04:49, Nanyong Sun a ?crit?:
> Remove redundant trampoline PGD for 64bit and add more comment
> for why 32bit systems need trampoline PGD.
>
> There was a patch and discussion similar to this,refer to
> the link [1][2].
>
> The trampoline PGD is redundant for 64bit systems because:
> 1. The early PGD covers the entire kernel mapping. Directly
> loading early PGD can achieve the result in boot stage.
> A more trampoline PGD makes code hard to understand.
> 2. Directly loading early PGD is safe in 64bit systems since
> the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx,
> which has a very big gap with RAM address.It won't fall into
> the corner case that 32bit system worrys.
> 3. Remove redundant trampoline PGD can benefit to code maintaince,
> because 64bit systems have more page table levels.For example:
> If we want to support SV48 which has 4 page table levels, we have
> to add a trampoline_pud and insert it before trampoline_pmd.
>
> Reference link:
> [1]https://lore.kernel.org/linux-riscv/[email protected]/
> [2]https://lkml.org/lkml/2019/3/28/147
>
> Signed-off-by: Nanyong Sun <[email protected]>
> Reviewed-by: Alexandre Ghiti <[email protected]>
I have not reviewed this patch yet! :)
> Reviewed-by: Kefeng Wang <[email protected]>
> Reviewed-by: weiyongjun <[email protected]>
> ---
> v2 changes:
> Adjust codes based on the review suggestions from Alex Ghiti,
> make codes more readable.
> ---
> arch/riscv/kernel/head.S | 46 ++++++++++++++++++++++++++--------------
> arch/riscv/mm/init.c | 16 ++++----------
> 2 files changed, 34 insertions(+), 28 deletions(-)
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index fce5184b22c3..3816aa5edc69 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -89,29 +89,52 @@ relocate:
> add ra, ra, a1
>
> /* Point stvec to virtual address of intruction after satp write */
> - la a2, 1f
> +#ifdef CONFIG_64BIT
> + la a2, load_done
> +#else
> + la a2, load_kernel_pgd
> +#endif
> add a2, a2, a1
> csrw CSR_TVEC, a2
>
> - /* Compute satp for kernel page tables, but don't load it yet */
> + /* Compute satp for kernel page tables.
> + * For 64bit systems, load it and trap to stvec.
> + * For 32bit systems, don't load it yet.
> + */
> srl a2, a0, PAGE_SHIFT
> li a1, SATP_MODE
> or a2, a2, a1
>
> /*
> + * Before writing satp, we need a full fence here because setup_vm() just
> + * wrote these PTEs and we need to ensure the new translations are in use.
> + */
> + sfence.vma
> +#ifndef CONFIG_64BIT
> + /*
> + * 32bit systems need firstly loading a trampoline to handle a corner
> + * case where load address range overlaps kernel virtual address range.
> * Load trampoline page directory, which will cause us to trap to
> - * stvec if VA != PA, or simply fall through if VA == PA. We need a
> - * full fence here because setup_vm() just wrote these PTEs and we need
> - * to ensure the new translations are in use.
> + * stvec if VA != PA, or simply fall through if VA == PA.
> */
> la a0, trampoline_pg_dir
> XIP_FIXUP_OFFSET a0
> srl a0, a0, PAGE_SHIFT
> or a0, a0, a1
> - sfence.vma
> csrw CSR_SATP, a0
> +#endif
> .align 2
> -1:
> +load_kernel_pgd:
> + /*
> + * Switch to kernel page tables. A full fence is necessary in order to
> + * avoid using the trampoline translations, which are only correct for
> + * the first superpage. Fetching the fence is guarnteed to work
> + * because that first superpage is translated the same way.
> + */
> + csrw CSR_SATP, a2
> + sfence.vma
> +
> +load_done:
> /* Set trap vector to spin forever to help debug */
> la a0, .Lsecondary_park
> csrw CSR_TVEC, a0
> @@ -122,15 +145,6 @@ relocate:
> la gp, __global_pointer$
> .option pop
>
> - /*
> - * Switch to kernel page tables. A full fence is necessary in order to
> - * avoid using the trampoline translations, which are only correct for
> - * the first superpage. Fetching the fence is guarnteed to work
> - * because that first superpage is translated the same way.
> - */
> - csrw CSR_SATP, a2
> - sfence.vma
> -
> ret
> #endif /* CONFIG_MMU */
> #ifdef CONFIG_SMP
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index ac48742fa6fc..306fcb2334fa 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
> EXPORT_SYMBOL(pfn_base);
>
> pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
> +#ifndef CONFIG_64BIT
> pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
> +#endif /* CONFIG_64BIT */
> static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
>
> pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>
> #ifdef CONFIG_XIP_KERNEL
> +#ifndef CONFIG_64BIT
> #define trampoline_pg_dir ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
> +#endif /* CONFIG_64BIT */
> #define fixmap_pte ((pte_t *)XIP_FIXUP(fixmap_pte))
> #define early_pg_dir ((pgd_t *)XIP_FIXUP(early_pg_dir))
> #endif /* CONFIG_XIP_KERNEL */
> @@ -300,13 +304,11 @@ static void __init create_pte_mapping(pte_t *ptep,
>
> #ifndef __PAGETABLE_PMD_FOLDED
>
> -static pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
> static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
> static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
> static pmd_t early_dtb_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
>
> #ifdef CONFIG_XIP_KERNEL
> -#define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
> #define fixmap_pmd ((pmd_t *)XIP_FIXUP(fixmap_pmd))
> #define early_pmd ((pmd_t *)XIP_FIXUP(early_pmd))
> #endif /* CONFIG_XIP_KERNEL */
> @@ -585,16 +587,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> /* Setup fixmap PMD */
> create_pmd_mapping(fixmap_pmd, FIXADDR_START,
> (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
> - /* Setup trampoline PGD and PMD */
> - create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
> - (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
> -#ifdef CONFIG_XIP_KERNEL
> - create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
> - kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
> -#else
> - create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
> - kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
> -#endif
> #else
> /* Setup trampoline PGD */
> create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
>
Hi Nanyong,
Le 28/07/2021 ? 13:55, Alex Ghiti a ?crit?:
>
>
> Le 28/07/2021 ? 04:49, Nanyong Sun a ?crit?:
>> Remove redundant trampoline PGD for 64bit and add more comment
>> for why 32bit systems need trampoline PGD.
>>
>> There was a patch and discussion similar to this,refer to
>> the link [1][2].
>>
>> The trampoline PGD is redundant for 64bit systems because:
>> 1. The early PGD covers the entire kernel mapping. Directly
>> loading early PGD can achieve the result in boot stage.
>> A more trampoline PGD makes code hard to understand.
>> 2. Directly loading early PGD is safe in 64bit systems since
>> the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx,
>> which has a very big gap with RAM address.It won't fall into
>> the corner case that 32bit system worrys.
>> 3. Remove redundant trampoline PGD can benefit to code maintaince,
>> because 64bit systems have more page table levels.For example:
>> If we want to support SV48 which has 4 page table levels, we have
>> to add a trampoline_pud and insert it before trampoline_pmd.
>>
>> Reference link:
>> [1]https://lore.kernel.org/linux-riscv/[email protected]/
>>
>> [2]https://lkml.org/lkml/2019/3/28/147
>>
>> Signed-off-by: Nanyong Sun <[email protected]>
>> Reviewed-by: Alexandre Ghiti <[email protected]>
>
> I have not reviewed this patch yet! :)
>
>> Reviewed-by: Kefeng Wang <[email protected]>
>> Reviewed-by: weiyongjun <[email protected]>
>> ---
>> v2 changes:
>> ?? Adjust codes based on the review suggestions from Alex Ghiti,
>> ?? make codes more readable.
>> ---
>> ? arch/riscv/kernel/head.S | 46 ++++++++++++++++++++++++++--------------
>> ? arch/riscv/mm/init.c???? | 16 ++++----------
>> ? 2 files changed, 34 insertions(+), 28 deletions(-)
>>
>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>> index fce5184b22c3..3816aa5edc69 100644
>> --- a/arch/riscv/kernel/head.S
>> +++ b/arch/riscv/kernel/head.S
>> @@ -89,29 +89,52 @@ relocate:
>> ????? add ra, ra, a1
>> ????? /* Point stvec to virtual address of intruction after satp write */
>> -??? la a2, 1f
>> +#ifdef CONFIG_64BIT
>> +??? la a2, load_done
>> +#else
>> +??? la a2, load_kernel_pgd
>> +#endif
>> ????? add a2, a2, a1
>> ????? csrw CSR_TVEC, a2
>> -??? /* Compute satp for kernel page tables, but don't load it yet */
>> +??? /* Compute satp for kernel page tables.
>> +???? * For 64bit systems, load it and trap to stvec.
>> +???? * For 32bit systems, don't load it yet.
>> +???? */
>> ????? srl a2, a0, PAGE_SHIFT
>> ????? li a1, SATP_MODE
>> ????? or a2, a2, a1
>> ????? /*
>> +???? * Before writing satp, we need a full fence here because
>> setup_vm() just
>> +???? * wrote these PTEs and we need to ensure the new translations
>> are in use.
>> +???? */
>> +??? sfence.vma
>> +#ifndef CONFIG_64BIT
>> +??? /*
>> +???? * 32bit systems need firstly loading a trampoline to handle a
>> corner
>> +???? * case where load address range overlaps kernel virtual address
>> range.
>> ?????? * Load trampoline page directory, which will cause us to trap to
>> -???? * stvec if VA != PA, or simply fall through if VA == PA.? We need a
>> -???? * full fence here because setup_vm() just wrote these PTEs and
>> we need
>> -???? * to ensure the new translations are in use.
>> +???? * stvec if VA != PA, or simply fall through if VA == PA.
>> ?????? */
>> ????? la a0, trampoline_pg_dir
>> ????? XIP_FIXUP_OFFSET a0
>> ????? srl a0, a0, PAGE_SHIFT
>> ????? or a0, a0, a1
>> -??? sfence.vma
>> ????? csrw CSR_SATP, a0
>> +#endif
>> ? .align 2
>> -1:
>> +load_kernel_pgd:
>> +??????? /*
>> +???????? * Switch to kernel page tables.? A full fence is necessary
>> in order to
>> +???????? * avoid using the trampoline translations, which are only
>> correct for
>> +???????? * the first superpage.? Fetching the fence is guarnteed to work
>> +???????? * because that first superpage is translated the same way.
>> +???????? */
>> +??????? csrw CSR_SATP, a2
>> +??????? sfence.vma
>> +
>> +load_done:
>> ????? /* Set trap vector to spin forever to help debug */
>> ????? la a0, .Lsecondary_park
>> ????? csrw CSR_TVEC, a0
I suppose stvec was set this way to catch any problem with early_pg_dir,
you moved that and then this defeats this original purpose.
>> @@ -122,15 +145,6 @@ relocate:
>> ????? la gp, __global_pointer$
>> ? .option pop
>> -??? /*
>> -???? * Switch to kernel page tables.? A full fence is necessary in
>> order to
>> -???? * avoid using the trampoline translations, which are only
>> correct for
>> -???? * the first superpage.? Fetching the fence is guarnteed to work
>> -???? * because that first superpage is translated the same way.
>> -???? */
>> -??? csrw CSR_SATP, a2
>> -??? sfence.vma
>> -
>> ????? ret
>> ? #endif /* CONFIG_MMU */
>> ? #ifdef CONFIG_SMP
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index ac48742fa6fc..306fcb2334fa 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
>> ? EXPORT_SYMBOL(pfn_base);
>> ? pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>> +#ifndef CONFIG_64BIT
>> ? pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>> +#endif /* CONFIG_64BIT */
As stated in Documentation/process/coding-style.rst, it is better to use
__maybe_unused rather than #ifdefs.
>> ? static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
>> ? pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>> ? #ifdef CONFIG_XIP_KERNEL
>> +#ifndef CONFIG_64BIT
>> ? #define trampoline_pg_dir????? ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
>> +#endif /* CONFIG_64BIT */
Using __maybe_unused allows to remove those too.
>> ? #define fixmap_pte???????????? ((pte_t *)XIP_FIXUP(fixmap_pte))
>> ? #define early_pg_dir?????????? ((pgd_t *)XIP_FIXUP(early_pg_dir))
>> ? #endif /* CONFIG_XIP_KERNEL */
>> @@ -300,13 +304,11 @@ static void __init create_pte_mapping(pte_t *ptep,
>> ? #ifndef __PAGETABLE_PMD_FOLDED
>> -static pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
>> ? static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
>> ? static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
>> ? static pmd_t early_dtb_pmd[PTRS_PER_PMD] __initdata
>> __aligned(PAGE_SIZE);
>> ? #ifdef CONFIG_XIP_KERNEL
>> -#define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
>> ? #define fixmap_pmd???? ((pmd_t *)XIP_FIXUP(fixmap_pmd))
>> ? #define early_pmd????? ((pmd_t *)XIP_FIXUP(early_pmd))
>> ? #endif /* CONFIG_XIP_KERNEL */
>> @@ -585,16 +587,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>> ????? /* Setup fixmap PMD */
>> ????? create_pmd_mapping(fixmap_pmd, FIXADDR_START,
>> ???????????????? (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
>> -??? /* Setup trampoline PGD and PMD */
>> -??? create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
>> -?????????????? (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
>> -#ifdef CONFIG_XIP_KERNEL
>> -??? create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>> -?????????????? kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
>> -#else
>> -??? create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>> -?????????????? kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
>> -#endif
>> ? #else
>> ????? /* Setup trampoline PGD */
>> ????? create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
>>
>
Overall this version adds more complexity to assembly code than I
thought, but I don't see any way to improve that (which does not mean
there isn't!).
Thanks,
Alex
> _______________________________________________
> linux-riscv mailing list
> [email protected]
> http://lists.infradead.org/mailman/listinfo/linux-riscv
On Mon, 02 Aug 2021 05:43:02 PDT (-0700), [email protected] wrote:
> Hi Nanyong,
>
> Le 28/07/2021 à 13:55, Alex Ghiti a écrit :
>>
>>
>> Le 28/07/2021 à 04:49, Nanyong Sun a écrit :
>>> Remove redundant trampoline PGD for 64bit and add more comment
>>> for why 32bit systems need trampoline PGD.
>>>
>>> There was a patch and discussion similar to this,refer to
>>> the link [1][2].
>>>
>>> The trampoline PGD is redundant for 64bit systems because:
>>> 1. The early PGD covers the entire kernel mapping. Directly
>>> loading early PGD can achieve the result in boot stage.
>>> A more trampoline PGD makes code hard to understand.
>>> 2. Directly loading early PGD is safe in 64bit systems since
>>> the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx,
>>> which has a very big gap with RAM address.It won't fall into
>>> the corner case that 32bit system worrys.
>>> 3. Remove redundant trampoline PGD can benefit to code maintaince,
>>> because 64bit systems have more page table levels.For example:
>>> If we want to support SV48 which has 4 page table levels, we have
>>> to add a trampoline_pud and insert it before trampoline_pmd.
>>>
>>> Reference link:
>>> [1]https://lore.kernel.org/linux-riscv/[email protected]/
>>>
>>> [2]https://lkml.org/lkml/2019/3/28/147
>>>
>>> Signed-off-by: Nanyong Sun <[email protected]>
>>> Reviewed-by: Alexandre Ghiti <[email protected]>
>>
>> I have not reviewed this patch yet! :)
>>
>>> Reviewed-by: Kefeng Wang <[email protected]>
>>> Reviewed-by: weiyongjun <[email protected]>
>>> ---
>>> v2 changes:
>>> Adjust codes based on the review suggestions from Alex Ghiti,
>>> make codes more readable.
>>> ---
>>> arch/riscv/kernel/head.S | 46 ++++++++++++++++++++++++++--------------
>>> arch/riscv/mm/init.c | 16 ++++----------
>>> 2 files changed, 34 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>>> index fce5184b22c3..3816aa5edc69 100644
>>> --- a/arch/riscv/kernel/head.S
>>> +++ b/arch/riscv/kernel/head.S
>>> @@ -89,29 +89,52 @@ relocate:
>>> add ra, ra, a1
>>> /* Point stvec to virtual address of intruction after satp write */
>>> - la a2, 1f
>>> +#ifdef CONFIG_64BIT
>>> + la a2, load_done
>>> +#else
>>> + la a2, load_kernel_pgd
>>> +#endif
>>> add a2, a2, a1
>>> csrw CSR_TVEC, a2
>>> - /* Compute satp for kernel page tables, but don't load it yet */
>>> + /* Compute satp for kernel page tables.
>>> + * For 64bit systems, load it and trap to stvec.
>>> + * For 32bit systems, don't load it yet.
>>> + */
>>> srl a2, a0, PAGE_SHIFT
>>> li a1, SATP_MODE
>>> or a2, a2, a1
>>> /*
>>> + * Before writing satp, we need a full fence here because
>>> setup_vm() just
>>> + * wrote these PTEs and we need to ensure the new translations
>>> are in use.
>>> + */
>>> + sfence.vma
>>> +#ifndef CONFIG_64BIT
>>> + /*
>>> + * 32bit systems need firstly loading a trampoline to handle a
>>> corner
>>> + * case where load address range overlaps kernel virtual address
>>> range.
>>> * Load trampoline page directory, which will cause us to trap to
>>> - * stvec if VA != PA, or simply fall through if VA == PA. We need a
>>> - * full fence here because setup_vm() just wrote these PTEs and
>>> we need
>>> - * to ensure the new translations are in use.
>>> + * stvec if VA != PA, or simply fall through if VA == PA.
>>> */
>>> la a0, trampoline_pg_dir
>>> XIP_FIXUP_OFFSET a0
>>> srl a0, a0, PAGE_SHIFT
>>> or a0, a0, a1
>>> - sfence.vma
>>> csrw CSR_SATP, a0
>>> +#endif
>>> .align 2
>>> -1:
>>> +load_kernel_pgd:
>>> + /*
>>> + * Switch to kernel page tables. A full fence is necessary
>>> in order to
>>> + * avoid using the trampoline translations, which are only
>>> correct for
>>> + * the first superpage. Fetching the fence is guarnteed to work
>>> + * because that first superpage is translated the same way.
>>> + */
>>> + csrw CSR_SATP, a2
>>> + sfence.vma
>>> +
>>> +load_done:
>>> /* Set trap vector to spin forever to help debug */
>>> la a0, .Lsecondary_park
>>> csrw CSR_TVEC, a0
>
>
> I suppose stvec was set this way to catch any problem with early_pg_dir,
> you moved that and then this defeats this original purpose.
Essentially.
The specific issue is that the JTAG debug spec is defined (or at least
was when I was using it, it's been years since I've needed to do that)
in terms of committed instructions. Thus if you end up in a position
where the processer is unable to commit an instruction you also lose the
ability to do anything meaningful with the debugger, thus essentially
locking up the system.
The most common way to end up in a situation where the processor is
unable to commit an instruction is to have a fault with an invalid trap
vector: maybe dangling from M-mode, the last boot, reset, whatever.
Then as soon as you take a trap the system locks up. Any trap before we
have a working trap handler is a bug, but it's way harder to debug
things when the debugger doesn't function.
There is of course no way to fundamentally prevent these sort of
no-commitable-instruction situations, but I got into the habbit of just
setting up a trivial trap entry point ASAP -- it probably took a dozen
rounds of trying to debug the debugger only to realize it was per spec
to hang, but that idiom eventually crept into pretty much everything.
Not sure if the debug spec is still written this way (or if debuggers
respect it), as I haven't had to use one in a while.
>
>
>>> @@ -122,15 +145,6 @@ relocate:
>>> la gp, __global_pointer$
>>> .option pop
>>> - /*
>>> - * Switch to kernel page tables. A full fence is necessary in
>>> order to
>>> - * avoid using the trampoline translations, which are only
>>> correct for
>>> - * the first superpage. Fetching the fence is guarnteed to work
>>> - * because that first superpage is translated the same way.
>>> - */
>>> - csrw CSR_SATP, a2
>>> - sfence.vma
>>> -
>>> ret
>>> #endif /* CONFIG_MMU */
>>> #ifdef CONFIG_SMP
>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>> index ac48742fa6fc..306fcb2334fa 100644
>>> --- a/arch/riscv/mm/init.c
>>> +++ b/arch/riscv/mm/init.c
>>> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
>>> EXPORT_SYMBOL(pfn_base);
>>> pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>> +#ifndef CONFIG_64BIT
>>> pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>> +#endif /* CONFIG_64BIT */
>
>
> As stated in Documentation/process/coding-style.rst, it is better to use
> __maybe_unused rather than #ifdefs.
>
>
>>> static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
>>> pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>>> #ifdef CONFIG_XIP_KERNEL
>>> +#ifndef CONFIG_64BIT
>>> #define trampoline_pg_dir ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
>>> +#endif /* CONFIG_64BIT */
>
>
> Using __maybe_unused allows to remove those too.
>
>
>>> #define fixmap_pte ((pte_t *)XIP_FIXUP(fixmap_pte))
>>> #define early_pg_dir ((pgd_t *)XIP_FIXUP(early_pg_dir))
>>> #endif /* CONFIG_XIP_KERNEL */
>>> @@ -300,13 +304,11 @@ static void __init create_pte_mapping(pte_t *ptep,
>>> #ifndef __PAGETABLE_PMD_FOLDED
>>> -static pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
>>> static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
>>> static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
>>> static pmd_t early_dtb_pmd[PTRS_PER_PMD] __initdata
>>> __aligned(PAGE_SIZE);
>>> #ifdef CONFIG_XIP_KERNEL
>>> -#define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
>>> #define fixmap_pmd ((pmd_t *)XIP_FIXUP(fixmap_pmd))
>>> #define early_pmd ((pmd_t *)XIP_FIXUP(early_pmd))
>>> #endif /* CONFIG_XIP_KERNEL */
>>> @@ -585,16 +587,6 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>> /* Setup fixmap PMD */
>>> create_pmd_mapping(fixmap_pmd, FIXADDR_START,
>>> (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
>>> - /* Setup trampoline PGD and PMD */
>>> - create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
>>> - (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
>>> -#ifdef CONFIG_XIP_KERNEL
>>> - create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>>> - kernel_map.xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
>>> -#else
>>> - create_pmd_mapping(trampoline_pmd, kernel_map.virt_addr,
>>> - kernel_map.phys_addr, PMD_SIZE, PAGE_KERNEL_EXEC);
>>> -#endif
>>> #else
>>> /* Setup trampoline PGD */
>>> create_pgd_mapping(trampoline_pg_dir, kernel_map.virt_addr,
>>>
>>
>
> Overall this version adds more complexity to assembly code than I
> thought, but I don't see any way to improve that (which does not mean
> there isn't!).
>
> Thanks,
>
> Alex
>
>
>> _______________________________________________
>> linux-riscv mailing list
>> [email protected]
>> http://lists.infradead.org/mailman/listinfo/linux-riscv
On 2021/8/14 6:08, Palmer Dabbelt wrote:
> On Mon, 02 Aug 2021 05:43:02 PDT (-0700), [email protected] wrote:
>> Hi Nanyong,
>>
>> Le 28/07/2021 à 13:55, Alex Ghiti a écrit :
>>>
>>>
>>> Le 28/07/2021 à 04:49, Nanyong Sun a écrit :
>>>> Remove redundant trampoline PGD for 64bit and add more comment
>>>> for why 32bit systems need trampoline PGD.
>>>>
>>>>
>>>> +load_kernel_pgd:
>>>> + /*
>>>> + * Switch to kernel page tables. A full fence is necessary
>>>> in order to
>>>> + * avoid using the trampoline translations, which are only
>>>> correct for
>>>> + * the first superpage. Fetching the fence is guarnteed
>>>> to work
>>>> + * because that first superpage is translated the same way.
>>>> + */
>>>> + csrw CSR_SATP, a2
>>>> + sfence.vma
>>>> +
>>>> +load_done:
>>>> /* Set trap vector to spin forever to help debug */
>>>> la a0, .Lsecondary_park
>>>> csrw CSR_TVEC, a0
>>
>>
>> I suppose stvec was set this way to catch any problem with early_pg_dir,
>> you moved that and then this defeats this original purpose.
>
Hi Alex,
I don't think so, before set early_pg_dir to satp, it's the
physical address world, we must set stvec as
the first place in virtual address world we want jump to. And I don't
think ".Lsecondary_park " can catch
problem of bad early_pg_dir, if the basic page table is wrong, CPU also
can not go to the virtual address stored in stvec correctly.
More, in the original code, before set trampoline_pg_dir, what if the
trampoline_pg_dir had a problem?
> Essentially.
>
> The specific issue is that the JTAG debug spec is defined (or at least
> was when I was using it, it's been years since I've needed to do that)
> in terms of committed instructions. Thus if you end up in a position
> where the processer is unable to commit an instruction you also lose
> the ability to do anything meaningful with the debugger, thus
> essentially locking up the system.
>
> The most common way to end up in a situation where the processor is
> unable to commit an instruction is to have a fault with an invalid
> trap vector: maybe dangling from M-mode, the last boot, reset,
> whatever. Then as soon as you take a trap the system locks up. Any
> trap before we have a working trap handler is a bug, but it's way
> harder to debug things when the debugger doesn't function.
>
> There is of course no way to fundamentally prevent these sort of
> no-commitable-instruction situations, but I got into the habbit of
> just setting up a trivial trap entry point ASAP -- it probably took a
> dozen rounds of trying to debug the debugger only to realize it was
> per spec to hang, but that idiom eventually crept into pretty much
> everything.
>
> Not sure if the debug spec is still written this way (or if debuggers
> respect it), as I haven't had to use one in a while.
>
>>
>>
>>>>
>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>> index ac48742fa6fc..306fcb2334fa 100644
>>>> --- a/arch/riscv/mm/init.c
>>>> +++ b/arch/riscv/mm/init.c
>>>> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
>>>> EXPORT_SYMBOL(pfn_base);
>>>> pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>> +#ifndef CONFIG_64BIT
>>>> pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>> +#endif /* CONFIG_64BIT */
>>
>>
>> As stated in Documentation/process/coding-style.rst, it is better to use
>> __maybe_unused rather than #ifdefs.
>>
>>
I'm afraid that __maybe_unused can not save one page memory here.
>>
>> Overall this version adds more complexity to assembly code than I
>> thought, but I don't see any way to improve that (which does not mean
>> there isn't!).
>>
>> Thanks,
>>
>> Alex
>>
Thanks for your review, let me figure out a better solution.
>>
>>> _______________________________________________
>>> linux-riscv mailing list
>>> [email protected]
>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>
> _______________________________________________
> linux-riscv mailing list
> [email protected]
> http://lists.infradead.org/mailman/listinfo/linux-riscv
Hi Nanyong,
Le 8/09/2021 à 08:42, Nanyong Sun a écrit :
>
> On 2021/8/14 6:08, Palmer Dabbelt wrote:
>> On Mon, 02 Aug 2021 05:43:02 PDT (-0700), [email protected] wrote:
>>> Hi Nanyong,
>>>
>>> Le 28/07/2021 à 13:55, Alex Ghiti a écrit :
>>>>
>>>>
>>>> Le 28/07/2021 à 04:49, Nanyong Sun a écrit :
>>>>> Remove redundant trampoline PGD for 64bit and add more comment
>>>>> for why 32bit systems need trampoline PGD.
>>>>>
>>>>>
>>>>> +load_kernel_pgd:
>>>>> + /*
>>>>> + * Switch to kernel page tables. A full fence is necessary
>>>>> in order to
>>>>> + * avoid using the trampoline translations, which are only
>>>>> correct for
>>>>> + * the first superpage. Fetching the fence is guarnteed
>>>>> to work
>>>>> + * because that first superpage is translated the same way.
>>>>> + */
>>>>> + csrw CSR_SATP, a2
>>>>> + sfence.vma
>>>>> +
>>>>> +load_done:
>>>>> /* Set trap vector to spin forever to help debug */
>>>>> la a0, .Lsecondary_park
>>>>> csrw CSR_TVEC, a0
>>>
>>>
>>> I suppose stvec was set this way to catch any problem with early_pg_dir,
>>> you moved that and then this defeats this original purpose.
>>
> Hi Alex,
>
> I don't think so, before set early_pg_dir to satp, it's the
> physical address world, we must set stvec as
>
> the first place in virtual address world we want jump to. And I don't
> think ".Lsecondary_park " can catch
>
> problem of bad early_pg_dir, if the basic page table is wrong, CPU also
> can not go to the virtual address stored in stvec correctly.
But I think then that it loops forever at the stvec address which allows
to know where the boot failed.
>
> More, in the original code, before set trampoline_pg_dir, what if the
> trampoline_pg_dir had a problem?
You're right but this debug 'feature' was not installed, I guess
somebody had a hard time at some point with the early page table and not
the trampoline :)
Anyway, I was just pointing that you 'broke' the current way things work
and unless this is for an explicit good reason, that should not happen.
>
>> Essentially.
>>
>> The specific issue is that the JTAG debug spec is defined (or at least
>> was when I was using it, it's been years since I've needed to do that)
>> in terms of committed instructions. Thus if you end up in a position
>> where the processer is unable to commit an instruction you also lose
>> the ability to do anything meaningful with the debugger, thus
>> essentially locking up the system.
>>
>> The most common way to end up in a situation where the processor is
>> unable to commit an instruction is to have a fault with an invalid
>> trap vector: maybe dangling from M-mode, the last boot, reset,
>> whatever. Then as soon as you take a trap the system locks up. Any
>> trap before we have a working trap handler is a bug, but it's way
>> harder to debug things when the debugger doesn't function.
>>
>> There is of course no way to fundamentally prevent these sort of
>> no-commitable-instruction situations, but I got into the habbit of
>> just setting up a trivial trap entry point ASAP -- it probably took a
>> dozen rounds of trying to debug the debugger only to realize it was
>> per spec to hang, but that idiom eventually crept into pretty much
>> everything.
>>
>> Not sure if the debug spec is still written this way (or if debuggers
>> respect it), as I haven't had to use one in a while.
>>
>>>
>>>
>>>>>
>>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>>> index ac48742fa6fc..306fcb2334fa 100644
>>>>> --- a/arch/riscv/mm/init.c
>>>>> +++ b/arch/riscv/mm/init.c
>>>>> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
>>>>> EXPORT_SYMBOL(pfn_base);
>>>>> pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>>> +#ifndef CONFIG_64BIT
>>>>> pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>>> +#endif /* CONFIG_64BIT */
>>>
>>>
>>> As stated in Documentation/process/coding-style.rst, it is better to use
>>> __maybe_unused rather than #ifdefs.
>>>
>>>
> I'm afraid that __maybe_unused can not save one page memory here.
What do you mean?
>>>
>>> Overall this version adds more complexity to assembly code than I
>>> thought, but I don't see any way to improve that (which does not mean
>>> there isn't!).
>>>
>>> Thanks,
>>>
>>> Alex
>>>
> Thanks for your review, let me figure out a better solution.
>>>
>>>> _______________________________________________
>>>> linux-riscv mailing list
>>>> [email protected]
>>>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>>
>> _______________________________________________
>> linux-riscv mailing list
>> [email protected]
>> http://lists.infradead.org/mailman/listinfo/linux-riscv
>
> _______________________________________________
> linux-riscv mailing list
> [email protected]
> http://lists.infradead.org/mailman/listinfo/linux-riscv
On 2021/9/8 16:56, Alex Ghiti wrote:
> Hi Nanyong,
>
> Le 8/09/2021 à 08:42, Nanyong Sun a écrit :
>>
>> On 2021/8/14 6:08, Palmer Dabbelt wrote:
>>> On Mon, 02 Aug 2021 05:43:02 PDT (-0700), [email protected] wrote:
>>>> Hi Nanyong,
>>>>
>>>> Le 28/07/2021 à 13:55, Alex Ghiti a écrit :
>>>>>
>>>>>
>>>>> Le 28/07/2021 à 04:49, Nanyong Sun a écrit :
>>>>>> Remove redundant trampoline PGD for 64bit and add more comment
>>>>>> for why 32bit systems need trampoline PGD.
>>>>>>
>>>>>>
>>>>>> +load_kernel_pgd:
>>>>>> + /*
>>>>>> + * Switch to kernel page tables. A full fence is necessary
>>>>>> in order to
>>>>>> + * avoid using the trampoline translations, which are only
>>>>>> correct for
>>>>>> + * the first superpage. Fetching the fence is guarnteed
>>>>>> to work
>>>>>> + * because that first superpage is translated the same way.
>>>>>> + */
>>>>>> + csrw CSR_SATP, a2
>>>>>> + sfence.vma
>>>>>> +
>>>>>> +load_done:
>>>>>> /* Set trap vector to spin forever to help debug */
>>>>>> la a0, .Lsecondary_park
>>>>>> csrw CSR_TVEC, a0
>>>>
>>>>
>>>> I suppose stvec was set this way to catch any problem with
>>>> early_pg_dir,
>>>> you moved that and then this defeats this original purpose.
>>>
>> Hi Alex,
>>
>> I don't think so, before set early_pg_dir to satp, it's the
>> physical address world, we must set stvec as
>>
>> the first place in virtual address world we want jump to. And I don't
>> think ".Lsecondary_park " can catch
>>
>> problem of bad early_pg_dir, if the basic page table is wrong, CPU
>> also can not go to the virtual address stored in stvec correctly.
>
> But I think then that it loops forever at the stvec address which
> allows to know where the boot failed.
If satp had a problem, then cpu can not fetch instruction where stvec
pointing to, as what palmer said: if you end up in a position where the
processer is unable to commit an instruction you also
lose the ability to do anything meaningful with the debugger, thus
essentially locking up the system.
>
>>
>> More, in the original code, before set trampoline_pg_dir, what if the
>> trampoline_pg_dir had a problem?
>
> You're right but this debug 'feature' was not installed, I guess
> somebody had a hard time at some point with the early page table and
> not the trampoline :)
>
> Anyway, I was just pointing that you 'broke' the current way things
> work and unless this is for an explicit good reason, that should not
> happen.
>
The design logic is: at the first time cpu convert to virtual address
world from physical world, actually stvec is a real "trampoline", it
can not be set
as a pointer to spin trap, it should be set to the first place in
virtual world where we wanna go. After that, then we set stvec as a spin
trap to catch
any problem in later running.
So, for 64bit system, if we want to delete trampoline_pg_dir, the
design principle is not broken here. For 32bit system, I really need
change back.
>>
>>> Essentially.
>>>
>>> The specific issue is that the JTAG debug spec is defined (or at
>>> least was when I was using it, it's been years since I've needed to
>>> do that) in terms of committed instructions. Thus if you end up in
>>> a position where the processer is unable to commit an instruction
>>> you also lose the ability to do anything meaningful with the
>>> debugger, thus essentially locking up the system.
>>>
>>> The most common way to end up in a situation where the processor is
>>> unable to commit an instruction is to have a fault with an invalid
>>> trap vector: maybe dangling from M-mode, the last boot, reset,
>>> whatever. Then as soon as you take a trap the system locks up. Any
>>> trap before we have a working trap handler is a bug, but it's way
>>> harder to debug things when the debugger doesn't function.
>>>
>>> There is of course no way to fundamentally prevent these sort of
>>> no-commitable-instruction situations, but I got into the habbit of
>>> just setting up a trivial trap entry point ASAP -- it probably took
>>> a dozen rounds of trying to debug the debugger only to realize it
>>> was per spec to hang, but that idiom eventually crept into pretty
>>> much everything.
>>>
>>> Not sure if the debug spec is still written this way (or if
>>> debuggers respect it), as I haven't had to use one in a while.
>>>
>>>>
>>>>
>>>>>>
>>>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>>>> index ac48742fa6fc..306fcb2334fa 100644
>>>>>> --- a/arch/riscv/mm/init.c
>>>>>> +++ b/arch/riscv/mm/init.c
>>>>>> @@ -219,13 +219,17 @@ unsigned long pfn_base __ro_after_init;
>>>>>> EXPORT_SYMBOL(pfn_base);
>>>>>> pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>>>> +#ifndef CONFIG_64BIT
>>>>>> pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>>>>>> +#endif /* CONFIG_64BIT */
>>>>
>>>>
>>>> As stated in Documentation/process/coding-style.rst, it is better
>>>> to use
>>>> __maybe_unused rather than #ifdefs.
>>>>
>>>>
>> I'm afraid that __maybe_unused can not save one page memory here.
>
> What do you mean?
>
I mean trampoline_pg_dir cost 4096 bytes here and __maybe_unused only
tell compiler don't raise a warning, but it still cost memory.