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In-Reply-To: <20230125151041.ijhjqswqiwmrzljd@orel>
From:   Alexandre Ghiti <alexghiti@rivosinc.com>
Date:   Fri, 27 Jan 2023 09:45:21 +0100
Message-ID: <CAHVXubjR8AsZhMz59goxfmf8LmA4bjePKUx=AkvmbqoF42tzmA@mail.gmail.com>
Subject: Re: [PATCH v4] riscv: Use PUD/P4D/PGD pages for the linear mapping
To:     Andrew Jones <ajones@ventanamicro.com>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>, Guo Ren <guoren@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, linux-arch@vger.kernel.org,
        Rob Herring <robh@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Hey Andrew,

Sorry about the delay.

On Wed, Jan 25, 2023 at 4:10 PM Andrew Jones <ajones@ventanamicro.com> wrote:
>
> On Wed, Jan 25, 2023 at 01:12:49PM +0100, Alexandre Ghiti wrote:
> > On Wed, Jan 25, 2023 at 11:41 AM Andrew Jones <ajones@ventanamicro.com> wrote:
> > >
> > > On Mon, Jan 23, 2023 at 03:25:54PM +0100, Andrew Jones wrote:
> > > > On Mon, Jan 23, 2023 at 12:28:02PM +0100, Alexandre Ghiti wrote:
> > > > > During the early page table creation, we used to set the mapping for
> > > > > PAGE_OFFSET to the kernel load address: but the kernel load address is
> > > > > always offseted by PMD_SIZE which makes it impossible to use PUD/P4D/PGD
> > > > > pages as this physical address is not aligned on PUD/P4D/PGD size (whereas
> > > > > PAGE_OFFSET is).
> > > > >
> > > > > But actually we don't have to establish this mapping (ie set va_pa_offset)
> > > > > that early in the boot process because:
> > > > >
> > > > > - first, setup_vm installs a temporary kernel mapping and among other
> > > > >   things, discovers the system memory,
> > > > > - then, setup_vm_final creates the final kernel mapping and takes
> > > > >   advantage of the discovered system memory to create the linear
> > > > >   mapping.
> > > > >
> > > > > During the first phase, we don't know the start of the system memory and
> > > > > then until the second phase is finished, we can't use the linear mapping at
> > > > > all and phys_to_virt/virt_to_phys translations must not be used because it
> > > > > would result in a different translation from the 'real' one once the final
> > > > > mapping is installed.
> > > > >
> > > > > So here we simply delay the initialization of va_pa_offset to after the
> > > > > system memory discovery. But to make sure noone uses the linear mapping
> > > > > before, we add some guard in the DEBUG_VIRTUAL config.
> > > > >
> > > > > Finally we can use PUD/P4D/PGD hugepages when possible, which will result
> > > > > in a better TLB utilization.
> > > > >
> > > > > Note that we rely on the firmware to protect itself using PMP.
> > > > >
> > > > > Acked-by: Rob Herring <robh@kernel.org> # DT bits
> > > > > Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> > > > > ---
> > > > >
> > > > > v4:
> > > > > - Rebase on top of v6.2-rc3, as noted by Conor
> > > > > - Add Acked-by Rob
> > > > >
> > > > > v3:
> > > > > - Change the comment about initrd_start VA conversion so that it fits
> > > > >   ARM64 and RISCV64 (and others in the future if needed), as suggested
> > > > >   by Rob
> > > > >
> > > > > v2:
> > > > > - Add a comment on why RISCV64 does not need to set initrd_start/end that
> > > > >   early in the boot process, as asked by Rob
> > > > >
> > > > >  arch/riscv/include/asm/page.h | 16 ++++++++++++++++
> > > > >  arch/riscv/mm/init.c          | 25 +++++++++++++++++++------
> > > > >  arch/riscv/mm/physaddr.c      | 16 ++++++++++++++++
> > > > >  drivers/of/fdt.c              | 11 ++++++-----
> > > > >  4 files changed, 57 insertions(+), 11 deletions(-)
> > > > >
> > > > > diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> > > > > index 9f432c1b5289..7fe84c89e572 100644
> > > > > --- a/arch/riscv/include/asm/page.h
> > > > > +++ b/arch/riscv/include/asm/page.h
> > > > > @@ -90,6 +90,14 @@ typedef struct page *pgtable_t;
> > > > >  #define PTE_FMT "%08lx"
> > > > >  #endif
> > > > >
> > > > > +#ifdef CONFIG_64BIT
> > > > > +/*
> > > > > + * We override this value as its generic definition uses __pa too early in
> > > > > + * the boot process (before kernel_map.va_pa_offset is set).
> > > > > + */
> > > > > +#define MIN_MEMBLOCK_ADDR      0
> > > > > +#endif
> > > > > +
> > > > >  #ifdef CONFIG_MMU
> > > > >  extern unsigned long riscv_pfn_base;
> > > > >  #define ARCH_PFN_OFFSET            (riscv_pfn_base)
> > > > > @@ -122,7 +130,11 @@ extern phys_addr_t phys_ram_base;
> > > > >  #define is_linear_mapping(x)       \
> > > > >     ((x) >= PAGE_OFFSET && (!IS_ENABLED(CONFIG_64BIT) || (x) < PAGE_OFFSET + KERN_VIRT_SIZE))
> > > > >
> > > > > +#ifndef CONFIG_DEBUG_VIRTUAL
> > > > >  #define linear_mapping_pa_to_va(x) ((void *)((unsigned long)(x) + kernel_map.va_pa_offset))
> > > > > +#else
> > > > > +void *linear_mapping_pa_to_va(unsigned long x);
> > > > > +#endif
> > > > >  #define kernel_mapping_pa_to_va(y) ({                                      \
> > > > >     unsigned long _y = (unsigned long)(y);                                  \
> > > > >     (IS_ENABLED(CONFIG_XIP_KERNEL) && _y < phys_ram_base) ?                 \
> > > > > @@ -131,7 +143,11 @@ extern phys_addr_t phys_ram_base;
> > > > >     })
> > > > >  #define __pa_to_va_nodebug(x)              linear_mapping_pa_to_va(x)
> > > > >
> > > > > +#ifndef CONFIG_DEBUG_VIRTUAL
> > > > >  #define linear_mapping_va_to_pa(x) ((unsigned long)(x) - kernel_map.va_pa_offset)
> > > > > +#else
> > > > > +phys_addr_t linear_mapping_va_to_pa(unsigned long x);
> > > > > +#endif
> > > > >  #define kernel_mapping_va_to_pa(y) ({                                              \
> > > > >     unsigned long _y = (unsigned long)(y);                                  \
> > > > >     (IS_ENABLED(CONFIG_XIP_KERNEL) && _y < kernel_map.virt_addr + XIP_OFFSET) ? \
> > > > > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > > > > index 478d6763a01a..cc892ba9f787 100644
> > > > > --- a/arch/riscv/mm/init.c
> > > > > +++ b/arch/riscv/mm/init.c
> > > > > @@ -213,6 +213,14 @@ static void __init setup_bootmem(void)
> > > > >     phys_ram_end = memblock_end_of_DRAM();
> > > > >     if (!IS_ENABLED(CONFIG_XIP_KERNEL))
> > > > >             phys_ram_base = memblock_start_of_DRAM();
> > > > > +
> > > > > +   /*
> > > > > +    * Any use of __va/__pa before this point is wrong as we did not know the
> > > > > +    * start of DRAM before.
> > > > > +    */
> > > > > +   kernel_map.va_pa_offset = PAGE_OFFSET - phys_ram_base;
> > > > > +   riscv_pfn_base = PFN_DOWN(phys_ram_base);
> > > > > +
> > > > >     /*
> > > > >      * memblock allocator is not aware of the fact that last 4K bytes of
> > > > >      * the addressable memory can not be mapped because of IS_ERR_VALUE
> > > > > @@ -671,9 +679,16 @@ void __init create_pgd_mapping(pgd_t *pgdp,
> > > > >
> > > > >  static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
> > > > >  {
> > > > > -   /* Upgrade to PMD_SIZE mappings whenever possible */
> > > > > -   base &= PMD_SIZE - 1;
> > > > > -   if (!base && size >= PMD_SIZE)
> > > > > +   if (!(base & (PGDIR_SIZE - 1)) && size >= PGDIR_SIZE)
> > > > > +           return PGDIR_SIZE;
> > > > > +
> > > > > +   if (!(base & (P4D_SIZE - 1)) && size >= P4D_SIZE)
> > > > > +           return P4D_SIZE;
> > > > > +
> > > > > +   if (!(base & (PUD_SIZE - 1)) && size >= PUD_SIZE)
> > > > > +           return PUD_SIZE;
> > > > > +
> > > > > +   if (!(base & (PMD_SIZE - 1)) && size >= PMD_SIZE)
> > > > >             return PMD_SIZE;
> > > > >
> > > > >     return PAGE_SIZE;
> > > > > @@ -982,11 +997,9 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> > > > >     set_satp_mode();
> > > > >  #endif
> > > > >
> > > > > -   kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
> > > > > +   kernel_map.va_pa_offset = 0UL;
> > > > >     kernel_map.va_kernel_pa_offset = kernel_map.virt_addr - kernel_map.phys_addr;
> > > > >
> > > > > -   riscv_pfn_base = PFN_DOWN(kernel_map.phys_addr);
> > > > > -
> > > > >     /*
> > > > >      * The default maximal physical memory size is KERN_VIRT_SIZE for 32-bit
> > > > >      * kernel, whereas for 64-bit kernel, the end of the virtual address
> > > > > diff --git a/arch/riscv/mm/physaddr.c b/arch/riscv/mm/physaddr.c
> > > > > index 9b18bda74154..18706f457da7 100644
> > > > > --- a/arch/riscv/mm/physaddr.c
> > > > > +++ b/arch/riscv/mm/physaddr.c
> > > > > @@ -33,3 +33,19 @@ phys_addr_t __phys_addr_symbol(unsigned long x)
> > > > >     return __va_to_pa_nodebug(x);
> > > > >  }
> > > > >  EXPORT_SYMBOL(__phys_addr_symbol);
> > > > > +
> > > > > +phys_addr_t linear_mapping_va_to_pa(unsigned long x)
> > > > > +{
> > > > > +   BUG_ON(!kernel_map.va_pa_offset);
> > > > > +
> > > > > +   return ((unsigned long)(x) - kernel_map.va_pa_offset);
> > > > > +}
> > > > > +EXPORT_SYMBOL(linear_mapping_va_to_pa);
> > > > > +
> > > > > +void *linear_mapping_pa_to_va(unsigned long x)
> > > > > +{
> > > > > +   BUG_ON(!kernel_map.va_pa_offset);
> > > > > +
> > > > > +   return ((void *)((unsigned long)(x) + kernel_map.va_pa_offset));
> > > > > +}
> > > > > +EXPORT_SYMBOL(linear_mapping_pa_to_va);
> > > > > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > > > > index f08b25195ae7..58107bd56f8f 100644
> > > > > --- a/drivers/of/fdt.c
> > > > > +++ b/drivers/of/fdt.c
> > > > > @@ -891,12 +891,13 @@ const void * __init of_flat_dt_match_machine(const void *default_match,
> > > > >  static void __early_init_dt_declare_initrd(unsigned long start,
> > > > >                                        unsigned long end)
> > > > >  {
> > > > > -   /* ARM64 would cause a BUG to occur here when CONFIG_DEBUG_VM is
> > > > > -    * enabled since __va() is called too early. ARM64 does make use
> > > > > -    * of phys_initrd_start/phys_initrd_size so we can skip this
> > > > > -    * conversion.
> > > > > +   /*
> > > > > +    * __va() is not yet available this early on some platforms. In that
> > > > > +    * case, the platform uses phys_initrd_start/phys_initrd_size instead
> > > > > +    * and does the VA conversion itself.
> > > > >      */
> > > > > -   if (!IS_ENABLED(CONFIG_ARM64)) {
> > > > > +   if (!IS_ENABLED(CONFIG_ARM64) &&
> > > > > +       !(IS_ENABLED(CONFIG_RISCV) && IS_ENABLED(CONFIG_64BIT))) {
> > > >
> > > > There are now two architectures, so maybe it's time for a new config
> > > > symbol which would be selected by arm64 and riscv64 and then used here,
> > > > e.g.
> > > >
> > > >   if (!IS_ENABLED(CONFIG_NO_EARLY_LINEAR_MAP)) {
> > >
> > > I see v5 left this as it was. Any comment on this suggestion?
> >
> > Introducing a config for this only use case sounds excessive to me,
> > but I'll let Rob decide what he wants to see here.
>
> To me, the suggestion is less about trying to tidy up DT code and more
> about bringing this comment about arm64 and riscv64 not being able to
> use the linear map as early as other architectures up out of the
> depths of DT code. Seeing an architecture select something like
> NO_EARLY_LINEAR_MAP, which has a paragraph explaining what that
> means, may help avoid other early uses of __va() which may or may
> not fail quickly and cleanly with a BUG.
>

You're right, do you have some bandwidth for doing that?

Thanks,

Alex

> Thanks,
> drew