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Subject: Re: [RFC PATCH 4/7] riscv: Implement sv48 support
To:     Anup Patel <anup@brainfault.org>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Zong Li <zong.li@sifive.com>, Christoph Hellwig <hch@lst.de>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
References: <mhng-938dc4d2-df13-48cf-a53e-4db7f23d52a5@palmerdabbelt-glaptop1>
 <04fed5f4-799b-94f4-731b-81475f8a73ba@ghiti.fr>
 <CAAhSdy0cxa0dUE2CV-t6G=fYO-UMJCJ9g-Y=KTZep1p+bPt6_A@mail.gmail.com>
From:   Alex Ghiti <alex@ghiti.fr>
Message-ID: <d014cd26-fe6c-3534-bfaf-e07bc91ae25e@ghiti.fr>
Date:   Wed, 8 Apr 2020 00:39:24 -0400
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Hi Anup,

On 4/7/20 1:56 AM, Anup Patel wrote:
> On Tue, Apr 7, 2020 at 10:44 AM Alex Ghiti <alex@ghiti.fr> wrote:
>>
>>
>> On 4/3/20 11:53 AM, Palmer Dabbelt wrote:
>>> On Sun, 22 Mar 2020 04:00:25 PDT (-0700), alex@ghiti.fr wrote:
>>>> By adding a new 4th level of page table, give the possibility to 64bit
>>>> kernel to address 2^48 bytes of virtual address: in practice, that
>>>> roughly
>>>> offers ~160TB of virtual address space to userspace and allows up to 64TB
>>>> of physical memory.
>>>>
>>>> By default, the kernel will try to boot with a 4-level page table. If the
>>>> underlying hardware does not support it, we will automatically
>>>> fallback to
>>>> a standard 3-level page table by folding the new PUD level into PGDIR
>>>> level.
>>>>
>>>> Early page table preparation is too early in the boot process to use any
>>>> device-tree entry, then in order to detect HW capabilities at runtime, we
>>>> use SATP feature that ignores writes with an unsupported mode. The
>>>> current
>>>> mode used by the kernel is then made available through cpuinfo.
>>>
>>> Ya, I think that's the right way to go about this.  There's no reason to
>>> rely on duplicate DT mechanisms for things the ISA defines for us.
>>>
>>>>
>>>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>>>> ---
>>>>   arch/riscv/Kconfig                  |   6 +-
>>>>   arch/riscv/include/asm/csr.h        |   3 +-
>>>>   arch/riscv/include/asm/fixmap.h     |   1 +
>>>>   arch/riscv/include/asm/page.h       |  15 +++-
>>>>   arch/riscv/include/asm/pgalloc.h    |  36 ++++++++
>>>>   arch/riscv/include/asm/pgtable-64.h |  98 ++++++++++++++++++++-
>>>>   arch/riscv/include/asm/pgtable.h    |   5 +-
>>>>   arch/riscv/kernel/head.S            |  37 ++++++--
>>>>   arch/riscv/mm/context.c             |   4 +-
>>>>   arch/riscv/mm/init.c                | 128 +++++++++++++++++++++++++---
>>>>   10 files changed, 302 insertions(+), 31 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>>> index a475c78e66bc..79560e94cc7c 100644
>>>> --- a/arch/riscv/Kconfig
>>>> +++ b/arch/riscv/Kconfig
>>>> @@ -66,6 +66,7 @@ config RISCV
>>>>       select ARCH_HAS_GCOV_PROFILE_ALL
>>>>       select HAVE_COPY_THREAD_TLS
>>>>       select HAVE_ARCH_KASAN if MMU && 64BIT
>>>> +    select RELOCATABLE if 64BIT
>>>>
>>>>   config ARCH_MMAP_RND_BITS_MIN
>>>>       default 18 if 64BIT
>>>> @@ -104,7 +105,7 @@ config PAGE_OFFSET
>>>>       default 0xC0000000 if 32BIT && MAXPHYSMEM_2GB
>>>>       default 0x80000000 if 64BIT && !MMU
>>>>       default 0xffffffff80000000 if 64BIT && MAXPHYSMEM_2GB
>>>> -    default 0xffffffe000000000 if 64BIT && !MAXPHYSMEM_2GB
>>>> +    default 0xffffc00000000000 if 64BIT && !MAXPHYSMEM_2GB
>>>>
>>>>   config ARCH_FLATMEM_ENABLE
>>>>       def_bool y
>>>> @@ -148,8 +149,11 @@ config GENERIC_HWEIGHT
>>>>   config FIX_EARLYCON_MEM
>>>>       def_bool MMU
>>>>
>>>> +# On a 64BIT relocatable kernel, the 4-level page table is at runtime
>>>> folded
>>>> +# on a 3-level page table when sv48 is not supported.
>>>>   config PGTABLE_LEVELS
>>>>       int
>>>> +    default 4 if 64BIT && RELOCATABLE
>>>>       default 3 if 64BIT
>>>>       default 2
>>>
>>> I assume this means you're relying on relocation to move the kernel around
>>> independently of PAGE_OFFSET in order to fold in the missing page table
>>> level?
>>
>> Yes, relocation is needed to fallback to 3-level and move PAGE_OFFSET
>> accordingly.
>>
>>> That seems reasonable, but it does impose a performance penalty as
>>> relocatable
>>> kernels necessitate slower generated code.  Additionally, there will
>>> likely be
>>> a performance penalty due to the extra memory access on TLB misses that is
>>> unnecessary for workloads that don't necessitate the longer VA width on
>>> machines that support it.
>>
>> Sorry, I had no time to answer your previous mail regarding performance:
>> I have no number. But the only penalty caused by this patchset on
>> 3-level page table is the check in page table management functions to
>> know if 4-level is activated or not. And as you said, the extra cost of
>> relocatable kernel that I had ignored since necessary anyway.
> 
> I guess we don't need relocation if we can avoid page table folding by
> detecting Sv48 mode very early in setup_vm(). Is there any other place
> where relocation would be required ?

Folding the 4th level is only a part of the problem, we also have to 
dynamically change the virtual address of the kernel: how can we achieve 
that without relocations ?

KASLR also uses relocations, see Zong's recent patchset.

Thanks,

Alex

> 
> If we can totally avoid relocation then it will certainly help in performance.
> 
> Regards,
> Anup
> 
>>
>>>
>>> I think the best bet here would be to have a Kconfig option for the
>>> number of
>>> page table levels (which could be MAXPHYSMEM or a second partially free
>>> parameter) and then another boolean argument along the lines of "also
>>> support
>>> machines with smaller VA widths".  It seems best to turn on the largest VA
>>> width and support for folding by default, as I assume that's what
>>> distros would
>>> do.
>>
>> I'm not a big fan of a new Kconfig option to allow people to have a
>> 3-level page table because that implies maintaining a new kernel, even
>> for us, having to compile 2 kernels each time we change something to mm
>> code will be painful.
>>
>> I have just reviewed Zong's KASLR patchset: he needs to parse the dtb to
>> find out the reserved regions in order to not override one of them when
>> copying the kernel to its new destination. And after that, he loops back
>> to setup_vm to re-create the mapping to the new kernel.
>> If that's the way we take for KASLR, we can follow the same path here:
>> boot with 4-level by default, go to check what is wanted in the device
>> tree and if it is 3-level, loop back to setup_vm.
>>
>>>
>>> I didn't really look closely at the rest of this, but it generally
>>> smells OK.
>>> The diff will need to be somewhat different for the next version, anyway :)
>>>
>>> Thanks for doing this!
>>>
>>>> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
>>>> index 435b65532e29..3828d55af85e 100644
>>>> --- a/arch/riscv/include/asm/csr.h
>>>> +++ b/arch/riscv/include/asm/csr.h
>>>> @@ -40,11 +40,10 @@
>>>>   #ifndef CONFIG_64BIT
>>>>   #define SATP_PPN    _AC(0x003FFFFF, UL)
>>>>   #define SATP_MODE_32    _AC(0x80000000, UL)
>>>> -#define SATP_MODE    SATP_MODE_32
>>>>   #else
>>>>   #define SATP_PPN    _AC(0x00000FFFFFFFFFFF, UL)
>>>>   #define SATP_MODE_39    _AC(0x8000000000000000, UL)
>>>> -#define SATP_MODE    SATP_MODE_39
>>>> +#define SATP_MODE_48    _AC(0x9000000000000000, UL)
>>>>   #endif
>>>>
>>>>   /* Exception cause high bit - is an interrupt if set */
>>>> diff --git a/arch/riscv/include/asm/fixmap.h
>>>> b/arch/riscv/include/asm/fixmap.h
>>>> index 42d2c42f3cc9..26e7799c5675 100644
>>>> --- a/arch/riscv/include/asm/fixmap.h
>>>> +++ b/arch/riscv/include/asm/fixmap.h
>>>> @@ -27,6 +27,7 @@ enum fixed_addresses {
>>>>       FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
>>>>       FIX_PTE,
>>>>       FIX_PMD,
>>>> +    FIX_PUD,
>>>>       FIX_EARLYCON_MEM_BASE,
>>>>       __end_of_fixed_addresses
>>>>   };
>>>> diff --git a/arch/riscv/include/asm/page.h
>>>> b/arch/riscv/include/asm/page.h
>>>> index 691f2f9ded2f..f1a26a0690ef 100644
>>>> --- a/arch/riscv/include/asm/page.h
>>>> +++ b/arch/riscv/include/asm/page.h
>>>> @@ -32,11 +32,19 @@
>>>>    * physical memory (aligned on a page boundary).
>>>>    */
>>>>   #ifdef CONFIG_RELOCATABLE
>>>> -extern unsigned long kernel_virt_addr;
>>>>   #define PAGE_OFFSET        kernel_virt_addr
>>>> +
>>>> +#ifdef CONFIG_64BIT
>>>> +/*
>>>> + * By default, CONFIG_PAGE_OFFSET value corresponds to SV48 address
>>>> space so
>>>> + * define the PAGE_OFFSET value for SV39.
>>>> + */
>>>> +#define PAGE_OFFSET_L3        0xffffffe000000000
>>>> +#define PAGE_OFFSET_L4        _AC(CONFIG_PAGE_OFFSET, UL)
>>>> +#endif /* CONFIG_64BIT */
>>>>   #else
>>>>   #define PAGE_OFFSET        _AC(CONFIG_PAGE_OFFSET, UL)
>>>> -#endif
>>>> +#endif /* CONFIG_RELOCATABLE */
>>>>
>>>>   #define KERN_VIRT_SIZE        -PAGE_OFFSET
>>>>
>>>> @@ -104,6 +112,9 @@ extern unsigned long pfn_base;
>>>>
>>>>   extern unsigned long max_low_pfn;
>>>>   extern unsigned long min_low_pfn;
>>>> +#ifdef CONFIG_RELOCATABLE
>>>> +extern unsigned long kernel_virt_addr;
>>>> +#endif
>>>>
>>>>   #define __pa_to_va_nodebug(x)    ((void *)((unsigned long) (x) +
>>>> va_pa_offset))
>>>>   #define __va_to_pa_nodebug(x)    ((unsigned long)(x) - va_pa_offset)
>>>> diff --git a/arch/riscv/include/asm/pgalloc.h
>>>> b/arch/riscv/include/asm/pgalloc.h
>>>> index 3f601ee8233f..540eaa5a8658 100644
>>>> --- a/arch/riscv/include/asm/pgalloc.h
>>>> +++ b/arch/riscv/include/asm/pgalloc.h
>>>> @@ -36,6 +36,42 @@ static inline void pud_populate(struct mm_struct
>>>> *mm, pud_t *pud, pmd_t *pmd)
>>>>
>>>>       set_pud(pud, __pud((pfn << _PAGE_PFN_SHIFT) | _PAGE_TABLE));
>>>>   }
>>>> +
>>>> +static inline void p4d_populate(struct mm_struct *mm, p4d_t *p4d,
>>>> pud_t *pud)
>>>> +{
>>>> +    if (pgtable_l4_enabled) {
>>>> +        unsigned long pfn = virt_to_pfn(pud);
>>>> +
>>>> +        set_p4d(p4d, __p4d((pfn << _PAGE_PFN_SHIFT) | _PAGE_TABLE));
>>>> +    }
>>>> +}
>>>> +
>>>> +static inline void p4d_populate_safe(struct mm_struct *mm, p4d_t *p4d,
>>>> +                     pud_t *pud)
>>>> +{
>>>> +    if (pgtable_l4_enabled) {
>>>> +        unsigned long pfn = virt_to_pfn(pud);
>>>> +
>>>> +        set_p4d_safe(p4d,
>>>> +                 __p4d((pfn << _PAGE_PFN_SHIFT) | _PAGE_TABLE));
>>>> +    }
>>>> +}
>>>> +
>>>> +static inline pud_t *pud_alloc_one(struct mm_struct *mm, unsigned
>>>> long addr)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return (pud_t *)__get_free_page(
>>>> +                GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_ZERO);
>>>> +    return NULL;
>>>> +}
>>>> +
>>>> +static inline void pud_free(struct mm_struct *mm, pud_t *pud)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        free_page((unsigned long)pud);
>>>> +}
>>>> +
>>>> +#define __pud_free_tlb(tlb, pud, addr)  pud_free((tlb)->mm, pud)
>>>>   #endif /* __PAGETABLE_PMD_FOLDED */
>>>>
>>>>   #define pmd_pgtable(pmd)    pmd_page(pmd)
>>>> diff --git a/arch/riscv/include/asm/pgtable-64.h
>>>> b/arch/riscv/include/asm/pgtable-64.h
>>>> index b15f70a1fdfa..cc4ffbe778f3 100644
>>>> --- a/arch/riscv/include/asm/pgtable-64.h
>>>> +++ b/arch/riscv/include/asm/pgtable-64.h
>>>> @@ -8,16 +8,32 @@
>>>>
>>>>   #include <linux/const.h>
>>>>
>>>> -#define PGDIR_SHIFT     30
>>>> +extern bool pgtable_l4_enabled;
>>>> +
>>>> +#define PGDIR_SHIFT     (pgtable_l4_enabled ? 39 : 30)
>>>>   /* Size of region mapped by a page global directory */
>>>>   #define PGDIR_SIZE      (_AC(1, UL) << PGDIR_SHIFT)
>>>>   #define PGDIR_MASK      (~(PGDIR_SIZE - 1))
>>>>
>>>> +/* pud is folded into pgd in case of 3-level page table */
>>>> +#define PUD_SHIFT    30
>>>> +#define PUD_SIZE    (_AC(1, UL) << PUD_SHIFT)
>>>> +#define PUD_MASK    (~(PUD_SIZE - 1))
>>>> +
>>>>   #define PMD_SHIFT       21
>>>>   /* Size of region mapped by a page middle directory */
>>>>   #define PMD_SIZE        (_AC(1, UL) << PMD_SHIFT)
>>>>   #define PMD_MASK        (~(PMD_SIZE - 1))
>>>>
>>>> +/* Page Upper Directory entry */
>>>> +typedef struct {
>>>> +    unsigned long pud;
>>>> +} pud_t;
>>>> +
>>>> +#define pud_val(x)      ((x).pud)
>>>> +#define __pud(x)        ((pud_t) { (x) })
>>>> +#define PTRS_PER_PUD    (PAGE_SIZE / sizeof(pud_t))
>>>> +
>>>>   /* Page Middle Directory entry */
>>>>   typedef struct {
>>>>       unsigned long pmd;
>>>> @@ -25,7 +41,6 @@ typedef struct {
>>>>
>>>>   #define pmd_val(x)      ((x).pmd)
>>>>   #define __pmd(x)        ((pmd_t) { (x) })
>>>> -
>>>>   #define PTRS_PER_PMD    (PAGE_SIZE / sizeof(pmd_t))
>>>>
>>>>   static inline int pud_present(pud_t pud)
>>>> @@ -60,6 +75,16 @@ static inline void pud_clear(pud_t *pudp)
>>>>       set_pud(pudp, __pud(0));
>>>>   }
>>>>
>>>> +static inline pud_t pfn_pud(unsigned long pfn, pgprot_t prot)
>>>> +{
>>>> +    return __pud((pfn << _PAGE_PFN_SHIFT) | pgprot_val(prot));
>>>> +}
>>>> +
>>>> +static inline unsigned long _pud_pfn(pud_t pud)
>>>> +{
>>>> +    return pud_val(pud) >> _PAGE_PFN_SHIFT;
>>>> +}
>>>> +
>>>>   static inline unsigned long pud_page_vaddr(pud_t pud)
>>>>   {
>>>>       return (unsigned long)pfn_to_virt(pud_val(pud) >> _PAGE_PFN_SHIFT);
>>>> @@ -70,6 +95,15 @@ static inline struct page *pud_page(pud_t pud)
>>>>       return pfn_to_page(pud_val(pud) >> _PAGE_PFN_SHIFT);
>>>>   }
>>>>
>>>> +#define mm_pud_folded    mm_pud_folded
>>>> +static inline bool mm_pud_folded(struct mm_struct *mm)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return false;
>>>> +
>>>> +    return true;
>>>> +}
>>>> +
>>>>   #define pmd_index(addr) (((addr) >> PMD_SHIFT) & (PTRS_PER_PMD - 1))
>>>>
>>>>   static inline pmd_t *pmd_offset(pud_t *pud, unsigned long addr)
>>>> @@ -90,4 +124,64 @@ static inline unsigned long _pmd_pfn(pmd_t pmd)
>>>>   #define pmd_ERROR(e) \
>>>>       pr_err("%s:%d: bad pmd %016lx.\n", __FILE__, __LINE__, pmd_val(e))
>>>>
>>>> +#define pud_ERROR(e)    \
>>>> +    pr_err("%s:%d: bad pud %016lx.\n", __FILE__, __LINE__, pud_val(e))
>>>> +
>>>> +static inline void set_p4d(p4d_t *p4dp, p4d_t p4d)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        *p4dp = p4d;
>>>> +    else
>>>> +        set_pud((pud_t *)p4dp, (pud_t){ p4d_val(p4d) });
>>>> +}
>>>> +
>>>> +static inline int p4d_none(p4d_t p4d)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return (p4d_val(p4d) == 0);
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>> +static inline int p4d_present(p4d_t p4d)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return (p4d_val(p4d) & _PAGE_PRESENT);
>>>> +
>>>> +    return 1;
>>>> +}
>>>> +
>>>> +static inline int p4d_bad(p4d_t p4d)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return !p4d_present(p4d);
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>> +static inline void p4d_clear(p4d_t *p4d)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        set_p4d(p4d, __p4d(0));
>>>> +}
>>>> +
>>>> +static inline unsigned long p4d_page_vaddr(p4d_t p4d)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return (unsigned long)pfn_to_virt(
>>>> +                p4d_val(p4d) >> _PAGE_PFN_SHIFT);
>>>> +
>>>> +    return pud_page_vaddr((pud_t) { p4d_val(p4d) });
>>>> +}
>>>> +
>>>> +#define pud_index(addr) (((addr) >> PUD_SHIFT) & (PTRS_PER_PUD - 1))
>>>> +
>>>> +static inline pud_t *pud_offset(p4d_t *p4d, unsigned long address)
>>>> +{
>>>> +    if (pgtable_l4_enabled)
>>>> +        return (pud_t *)p4d_page_vaddr(*p4d) + pud_index(address);
>>>> +
>>>> +    return (pud_t *)p4d;
>>>> +}
>>>> +
>>>>   #endif /* _ASM_RISCV_PGTABLE_64_H */
>>>> diff --git a/arch/riscv/include/asm/pgtable.h
>>>> b/arch/riscv/include/asm/pgtable.h
>>>> index dce401eed1d3..06361db3f486 100644
>>>> --- a/arch/riscv/include/asm/pgtable.h
>>>> +++ b/arch/riscv/include/asm/pgtable.h
>>>> @@ -13,8 +13,7 @@
>>>>
>>>>   #ifndef __ASSEMBLY__
>>>>
>>>> -/* Page Upper Directory not used in RISC-V */
>>>> -#include <asm-generic/pgtable-nopud.h>
>>>> +#include <asm-generic/pgtable-nop4d.h>
>>>>   #include <asm/page.h>
>>>>   #include <asm/tlbflush.h>
>>>>   #include <linux/mm_types.h>
>>>> @@ -27,7 +26,7 @@
>>>>
>>>>   #ifdef CONFIG_MMU
>>>>   #ifdef CONFIG_64BIT
>>>> -#define VA_BITS        39
>>>> +#define VA_BITS        (pgtable_l4_enabled ? 48 : 39)
>>>>   #define PA_BITS        56
>>>>   #else
>>>>   #define VA_BITS        32
>>>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>>>> index 1c2fbefb8786..22617bd7477f 100644
>>>> --- a/arch/riscv/kernel/head.S
>>>> +++ b/arch/riscv/kernel/head.S
>>>> @@ -113,6 +113,8 @@ clear_bss_done:
>>>>       call setup_vm
>>>>   #ifdef CONFIG_MMU
>>>>       la a0, early_pg_dir
>>>> +    la a1, satp_mode
>>>> +    REG_L a1, (a1)
>>>>       call relocate
>>>>   #endif /* CONFIG_MMU */
>>>>
>>>> @@ -131,24 +133,28 @@ clear_bss_done:
>>>>   #ifdef CONFIG_MMU
>>>>   relocate:
>>>>   #ifdef CONFIG_RELOCATABLE
>>>> -    /* Relocate return address */
>>>> -    la a1, kernel_virt_addr
>>>> -    REG_L a1, 0(a1)
>>>> +    /*
>>>> +     * Relocate return address but save it in case 4-level page table is
>>>> +     * not supported.
>>>> +     */
>>>> +    mv s1, ra
>>>> +    la a3, kernel_virt_addr
>>>> +    REG_L a3, 0(a3)
>>>>   #else
>>>> -    li a1, PAGE_OFFSET
>>>> +    li a3, PAGE_OFFSET
>>>>   #endif
>>>>       la a2, _start
>>>> -    sub a1, a1, a2
>>>> -    add ra, ra, a1
>>>> +    sub a3, a3, a2
>>>> +    add ra, ra, a3
>>>>
>>>>       /* Point stvec to virtual address of intruction after satp write */
>>>>       la a2, 1f
>>>> -    add a2, a2, a1
>>>> +    add a2, a2, a3
>>>>       csrw CSR_TVEC, a2
>>>>
>>>> +    /* First try with a 4-level page table */
>>>>       /* Compute satp for kernel page tables, but don't load it yet */
>>>>       srl a2, a0, PAGE_SHIFT
>>>> -    li a1, SATP_MODE
>>>>       or a2, a2, a1
>>>>
>>>>       /*
>>>> @@ -162,6 +168,19 @@ relocate:
>>>>       or a0, a0, a1
>>>>       sfence.vma
>>>>       csrw CSR_SATP, a0
>>>> +#if defined(CONFIG_RELOCATABLE) && defined(CONFIG_64BIT)
>>>> +    /*
>>>> +     * If we fall through here, that means the HW does not support SV48.
>>>> +     * We need a 3-level page table then simply fold pud into pgd level
>>>> +     * and finally jump back to relocate with 3-level parameters.
>>>> +     */
>>>> +    call setup_vm_fold_pud
>>>> +
>>>> +    la a0, early_pg_dir
>>>> +    li a1, SATP_MODE_39
>>>> +    mv ra, s1
>>>> +    tail relocate
>>>> +#endif
>>>>   .align 2
>>>>   1:
>>>>       /* Set trap vector to spin forever to help debug */
>>>> @@ -213,6 +232,8 @@ relocate:
>>>>   #ifdef CONFIG_MMU
>>>>       /* Enable virtual memory and relocate to virtual address */
>>>>       la a0, swapper_pg_dir
>>>> +    la a1, satp_mode
>>>> +    REG_L a1, (a1)
>>>>       call relocate
>>>>   #endif
>>>>
>>>> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
>>>> index 613ec81a8979..152b423c02ea 100644
>>>> --- a/arch/riscv/mm/context.c
>>>> +++ b/arch/riscv/mm/context.c
>>>> @@ -9,6 +9,8 @@
>>>>   #include <asm/cacheflush.h>
>>>>   #include <asm/mmu_context.h>
>>>>
>>>> +extern uint64_t satp_mode;
>>>> +
>>>>   /*
>>>>    * When necessary, performs a deferred icache flush for the given MM
>>>> context,
>>>>    * on the local CPU.  RISC-V has no direct mechanism for instruction
>>>> cache
>>>> @@ -59,7 +61,7 @@ void switch_mm(struct mm_struct *prev, struct
>>>> mm_struct *next,
>>>>       cpumask_set_cpu(cpu, mm_cpumask(next));
>>>>
>>>>   #ifdef CONFIG_MMU
>>>> -    csr_write(CSR_SATP, virt_to_pfn(next->pgd) | SATP_MODE);
>>>> +    csr_write(CSR_SATP, virt_to_pfn(next->pgd) | satp_mode);
>>>>       local_flush_tlb_all();
>>>>   #endif
>>>>
>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>> index 18bbb426848e..ad96667d2ab6 100644
>>>> --- a/arch/riscv/mm/init.c
>>>> +++ b/arch/riscv/mm/init.c
>>>> @@ -24,6 +24,17 @@
>>>>
>>>>   #include "../kernel/head.h"
>>>>
>>>> +#ifdef CONFIG_64BIT
>>>> +uint64_t satp_mode = IS_ENABLED(CONFIG_MAXPHYSMEM_2GB) ?
>>>> +                SATP_MODE_39 : SATP_MODE_48;
>>>> +bool pgtable_l4_enabled = IS_ENABLED(CONFIG_MAXPHYSMEM_2GB) ? false :
>>>> true;
>>>> +#else
>>>> +uint64_t satp_mode = SATP_MODE_32;
>>>> +bool pgtable_l4_enabled = false;
>>>> +#endif
>>>> +EXPORT_SYMBOL(pgtable_l4_enabled);
>>>> +EXPORT_SYMBOL(satp_mode);
>>>> +
>>>>   unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
>>>>                               __page_aligned_bss;
>>>>   EXPORT_SYMBOL(empty_zero_page);
>>>> @@ -245,9 +256,12 @@ static void __init create_pte_mapping(pte_t *ptep,
>>>>
>>>>   #ifndef __PAGETABLE_PMD_FOLDED
>>>>
>>>> +pud_t trampoline_pud[PTRS_PER_PUD] __page_aligned_bss;
>>>>   pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
>>>> +pud_t fixmap_pud[PTRS_PER_PUD] __page_aligned_bss;
>>>>   pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
>>>>   pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
>>>> +pud_t early_pud[PTRS_PER_PUD] __initdata __aligned(PAGE_SIZE);
>>>>
>>>>   static pmd_t *__init get_pmd_virt(phys_addr_t pa)
>>>>   {
>>>> @@ -264,7 +278,8 @@ static phys_addr_t __init alloc_pmd(uintptr_t va)
>>>>       if (mmu_enabled)
>>>>           return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>>>>
>>>> -    BUG_ON((va - PAGE_OFFSET) >> PGDIR_SHIFT);
>>>> +    /* Only one PMD is available for early mapping */
>>>> +    BUG_ON((va - PAGE_OFFSET) >> PUD_SHIFT);
>>>>
>>>>       return (uintptr_t)early_pmd;
>>>>   }
>>>> @@ -296,19 +311,70 @@ static void __init create_pmd_mapping(pmd_t *pmdp,
>>>>       create_pte_mapping(ptep, va, pa, sz, prot);
>>>>   }
>>>>
>>>> -#define pgd_next_t        pmd_t
>>>> -#define alloc_pgd_next(__va)    alloc_pmd(__va)
>>>> -#define get_pgd_next_virt(__pa)    get_pmd_virt(__pa)
>>>> +static pud_t *__init get_pud_virt(phys_addr_t pa)
>>>> +{
>>>> +    if (mmu_enabled) {
>>>> +        clear_fixmap(FIX_PUD);
>>>> +        return (pud_t *)set_fixmap_offset(FIX_PUD, pa);
>>>> +    } else {
>>>> +        return (pud_t *)((uintptr_t)pa);
>>>> +    }
>>>> +}
>>>> +
>>>> +static phys_addr_t __init alloc_pud(uintptr_t va)
>>>> +{
>>>> +    if (mmu_enabled)
>>>> +        return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>>>> +
>>>> +    /* Only one PUD is available for early mapping */
>>>> +    BUG_ON((va - PAGE_OFFSET) >> PGDIR_SHIFT);
>>>> +
>>>> +    return (uintptr_t)early_pud;
>>>> +}
>>>> +
>>>> +static void __init create_pud_mapping(pud_t *pudp,
>>>> +                      uintptr_t va, phys_addr_t pa,
>>>> +                      phys_addr_t sz, pgprot_t prot)
>>>> +{
>>>> +    pmd_t *nextp;
>>>> +    phys_addr_t next_phys;
>>>> +    uintptr_t pud_index = pud_index(va);
>>>> +
>>>> +    if (sz == PUD_SIZE) {
>>>> +        if (pud_val(pudp[pud_index]) == 0)
>>>> +            pudp[pud_index] = pfn_pud(PFN_DOWN(pa), prot);
>>>> +        return;
>>>> +    }
>>>> +
>>>> +    if (pud_val(pudp[pud_index]) == 0) {
>>>> +        next_phys = alloc_pmd(va);
>>>> +        pudp[pud_index] = pfn_pud(PFN_DOWN(next_phys), PAGE_TABLE);
>>>> +        nextp = get_pmd_virt(next_phys);
>>>> +        memset(nextp, 0, PAGE_SIZE);
>>>> +    } else {
>>>> +        next_phys = PFN_PHYS(_pud_pfn(pudp[pud_index]));
>>>> +        nextp = get_pmd_virt(next_phys);
>>>> +    }
>>>> +
>>>> +    create_pmd_mapping(nextp, va, pa, sz, prot);
>>>> +}
>>>> +
>>>> +#define pgd_next_t        pud_t
>>>> +#define alloc_pgd_next(__va)    alloc_pud(__va)
>>>> +#define get_pgd_next_virt(__pa)    get_pud_virt(__pa)
>>>>   #define create_pgd_next_mapping(__nextp, __va, __pa, __sz, __prot)    \
>>>> -    create_pmd_mapping(__nextp, __va, __pa, __sz, __prot)
>>>> -#define fixmap_pgd_next        fixmap_pmd
>>>> +    create_pud_mapping(__nextp, __va, __pa, __sz, __prot)
>>>> +#define fixmap_pgd_next        (pgtable_l4_enabled ?            \
>>>> +            (uintptr_t)fixmap_pud : (uintptr_t)fixmap_pmd)
>>>> +#define trampoline_pgd_next    (pgtable_l4_enabled ?            \
>>>> +            (uintptr_t)trampoline_pud : (uintptr_t)trampoline_pmd)
>>>>   #else
>>>>   #define pgd_next_t        pte_t
>>>>   #define alloc_pgd_next(__va)    alloc_pte(__va)
>>>>   #define get_pgd_next_virt(__pa)    get_pte_virt(__pa)
>>>>   #define create_pgd_next_mapping(__nextp, __va, __pa, __sz, __prot)    \
>>>>       create_pte_mapping(__nextp, __va, __pa, __sz, __prot)
>>>> -#define fixmap_pgd_next        fixmap_pte
>>>> +#define fixmap_pgd_next        ((uintptr_t)fixmap_pte)
>>>>   #endif
>>>>
>>>>   static void __init create_pgd_mapping(pgd_t *pgdp,
>>>> @@ -319,6 +385,13 @@ static void __init create_pgd_mapping(pgd_t *pgdp,
>>>>       phys_addr_t next_phys;
>>>>       uintptr_t pgd_index = pgd_index(va);
>>>>
>>>> +#ifndef __PAGETABLE_PMD_FOLDED
>>>> +    if (!pgtable_l4_enabled) {
>>>> +        create_pud_mapping((pud_t *)pgdp, va, pa, sz, prot);
>>>> +        return;
>>>> +    }
>>>> +#endif
>>>> +
>>>>       if (sz == PGDIR_SIZE) {
>>>>           if (pgd_val(pgdp[pgd_index]) == 0)
>>>>               pgdp[pgd_index] = pfn_pgd(PFN_DOWN(pa), prot);
>>>> @@ -449,15 +522,22 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>
>>>>       /* Setup early PGD for fixmap */
>>>>       create_pgd_mapping(early_pg_dir, FIXADDR_START,
>>>> -               (uintptr_t)fixmap_pgd_next, PGDIR_SIZE, PAGE_TABLE);
>>>> +               fixmap_pgd_next, PGDIR_SIZE, PAGE_TABLE);
>>>>
>>>>   #ifndef __PAGETABLE_PMD_FOLDED
>>>> -    /* Setup fixmap PMD */
>>>> +    /* Setup fixmap PUD and PMD */
>>>> +    if (pgtable_l4_enabled)
>>>> +        create_pud_mapping(fixmap_pud, FIXADDR_START,
>>>> +               (uintptr_t)fixmap_pmd, PUD_SIZE, PAGE_TABLE);
>>>>       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, PAGE_OFFSET,
>>>> -               (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
>>>> +               trampoline_pgd_next, PGDIR_SIZE, PAGE_TABLE);
>>>> +    if (pgtable_l4_enabled)
>>>> +        create_pud_mapping(trampoline_pud, PAGE_OFFSET,
>>>> +               (uintptr_t)trampoline_pmd, PUD_SIZE, PAGE_TABLE);
>>>>       create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
>>>>                  load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
>>>>   #else
>>>> @@ -490,6 +570,29 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>       dtb_early_pa = dtb_pa;
>>>>   }
>>>>
>>>> +#if defined(CONFIG_RELOCATABLE) && defined(CONFIG_64BIT)
>>>> +/*
>>>> + * This function is called only if the current kernel is 64bit and
>>>> the HW
>>>> + * does not support sv48.
>>>> + */
>>>> +asmlinkage __init void setup_vm_fold_pud(void)
>>>> +{
>>>> +    pgtable_l4_enabled = false;
>>>> +    kernel_virt_addr = PAGE_OFFSET_L3;
>>>> +    satp_mode = SATP_MODE_39;
>>>> +
>>>> +    /*
>>>> +     * PTE/PMD levels do not need to be cleared as they are common
>>>> between
>>>> +     * 3- and 4-level page tables: the 30 least significant bits
>>>> +     * (2 * 9 + 12) are common.
>>>> +     */
>>>> +    memset(trampoline_pg_dir, 0, sizeof(pgd_t) * PTRS_PER_PGD);
>>>> +    memset(early_pg_dir, 0, sizeof(pgd_t) * PTRS_PER_PGD);
>>>> +
>>>> +    setup_vm(dtb_early_pa);
>>>> +}
>>>> +#endif
>>>> +
>>>>   static void __init setup_vm_final(void)
>>>>   {
>>>>       uintptr_t va, map_size;
>>>> @@ -525,12 +628,13 @@ static void __init setup_vm_final(void)
>>>>           }
>>>>       }
>>>>
>>>> -    /* Clear fixmap PTE and PMD mappings */
>>>> +    /* Clear fixmap page table mappings */
>>>>       clear_fixmap(FIX_PTE);
>>>>       clear_fixmap(FIX_PMD);
>>>> +    clear_fixmap(FIX_PUD);
>>>>
>>>>       /* Move to swapper page table */
>>>> -    csr_write(CSR_SATP, PFN_DOWN(__pa_symbol(swapper_pg_dir)) |
>>>> SATP_MODE);
>>>> +    csr_write(CSR_SATP, PFN_DOWN(__pa_symbol(swapper_pg_dir)) |
>>>> satp_mode);
>>>>       local_flush_tlb_all();
>>>>   }
>>>>   #else
>>
>> Alex