Re: [PATCH v9 3/3] riscv: Use PUD/P4D/PGD pages for the linear mapping

Andrew Jones
2023-03-27T09:39:33Z

On Fri, Mar 24, 2023 at 04:54:21PM +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:
> - this does not apply to rv32 as the kernel mapping lies in the linear
> mapping.
> - we rely on the firmware to protect itself using PMP.
>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Acked-by: Rob Herring <robh@kernel.org> # DT bits
> ---
> arch/riscv/include/asm/page.h | 16 ++++++++++
> arch/riscv/mm/init.c | 58 +++++++++++++++++++++++++++++++----
> arch/riscv/mm/physaddr.c | 16 ++++++++++
> drivers/of/fdt.c | 11 ++++---
> 4 files changed, 90 insertions(+), 11 deletions(-)
>
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index 8dc686f549b6..ea1a0e237211 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
> #define ARCH_PFN_OFFSET (PFN_DOWN((unsigned long)phys_ram_base))
> #else
> @@ -121,7 +129,11 @@ 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) ? \
> @@ -130,7 +142,11 @@ 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 3b37d8606920..f803671d18b2 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();
> +
> + /*
> + * In 64-bit, any use of __va/__pa before this point is wrong as we
> + * did not know the start of DRAM before.
> + */
> + if (IS_ENABLED(CONFIG_64BIT))
> + kernel_map.va_pa_offset = PAGE_OFFSET - 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
> @@ -667,9 +675,16 @@ 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;
> @@ -978,11 +993,22 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> set_satp_mode();
> #endif
>
> - kernel_map.va_pa_offset = PAGE_OFFSET - kernel_map.phys_addr;
> + /*
> + * In 64-bit, we defer the setup of va_pa_offset to setup_bootmem,
> + * where we have the system memory layout: this allows us to align
> + * the physical and virtual mappings and then make use of PUD/P4D/PGD
> + * for the linear mapping. This is only possible because the kernel
> + * mapping lies outside the linear mapping.
> + * In 32-bit however, as the kernel resides in the linear mapping,
> + * setup_vm_final can not change the mapping established here,
> + * otherwise the same kernel addresses would get mapped to different
> + * physical addresses (if the start of dram is different from the
> + * kernel physical address start).
> + */
> + kernel_map.va_pa_offset = IS_ENABLED(CONFIG_64BIT) ?
> + 0UL : PAGE_OFFSET - kernel_map.phys_addr;
> kernel_map.va_kernel_pa_offset = kernel_map.virt_addr - kernel_map.phys_addr;
>
> - phys_ram_base = 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
> @@ -1106,6 +1132,17 @@ static void __init create_linear_mapping_page_table(void)
> phys_addr_t start, end;
> u64 i;
>
> +#ifdef CONFIG_STRICT_KERNEL_RWX
> + phys_addr_t ktext_start = __pa_symbol(_start);
> + phys_addr_t ktext_size = __init_data_begin - _start;
> + phys_addr_t krodata_start = __pa_symbol(__start_rodata);
> + phys_addr_t krodata_size = _data - __start_rodata;
> +
> + /* Isolate kernel text and rodata so they don't get mapped with a PUD */
> + memblock_mark_nomap(ktext_start, ktext_size);
> + memblock_mark_nomap(krodata_start, krodata_size);
> +#endif
> +
> /* Map all memory banks in the linear mapping */
> for_each_mem_range(i, &start, &end) {
> if (start >= end)
> @@ -1118,6 +1155,15 @@ static void __init create_linear_mapping_page_table(void)
>
> create_linear_mapping_range(start, end);
> }
> +
> +#ifdef CONFIG_STRICT_KERNEL_RWX
> + create_linear_mapping_range(ktext_start, ktext_start + ktext_size);
> + create_linear_mapping_range(krodata_start,
> + krodata_start + krodata_size);

Just for my own education, it looks to me like the rodata is left writable
until the end of start_kernel(), when mark_rodata_ro() is called. Is that
correct?

Thanks,
drew

> +
> + memblock_clear_nomap(ktext_start, ktext_size);
> + memblock_clear_nomap(krodata_start, krodata_size);
> +#endif
> }
>
> static void __init setup_vm_final(void)
> 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 d1a68b6d03b3..d14735a81301 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -887,12 +887,13 @@ 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))) {
> initrd_start = (unsigned long)__va(start);
> initrd_end = (unsigned long)__va(end);
> initrd_below_start_ok = 1;
> --
> 2.37.2
>