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SubjectRe: [PATCH -next] riscv: mm: remove redundant trampoline PGD for 64bit
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Hi Nanyong,

Le 27/05/2021 à 16:48, 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.

I do agree with your last point about code maintenance and this would be
a welcome improvement before I respin my sv48 series.

Some comments below though.

>
> Reference link:
> [1]https://lore.kernel.org/linux-riscv/20190325092234.5451-4-anup.patel@wdc.com/
> [2]https://lkml.org/lkml/2019/3/28/147
>
> Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
> ---
> arch/riscv/kernel/head.S | 13 +++++++++++--
> arch/riscv/mm/init.c | 21 +++++++--------------
> 2 files changed, 18 insertions(+), 16 deletions(-)
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 89cc58ab52b4..1897b17c5fcc 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -93,12 +93,18 @@ relocate:
> add a2, a2, a1
> csrw CSR_TVEC, a2

This is not needed in 64-bit then.

>
> - /* Compute satp for kernel page tables, but don't load it yet */
> + /* Compute satp for kernel page tables */
> srl a2, a0, PAGE_SHIFT
> li a1, SATP_MODE
> or a2, a2, a1
> -
> +#ifdef CONFIG_64BIT
> + /* Load kernel page directory */
> + sfence.vma
> + csrw CSR_SATP, a2
> +#else
> /*
> + * 32bit system need a trampoline to handle a corner case where

s/system/systems or s/need/needs.

> + * 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
> @@ -110,6 +116,7 @@ relocate:
> or a0, a0, a1
> sfence.vma
> csrw CSR_SATP, a0
> +#endif /* CONFIG_64BIT */
> .align 2
> 1:
> /* Set trap vector to spin forever to help debug */
> @@ -122,6 +129,7 @@ relocate:
> la gp, __global_pointer$
> .option pop
>
> +#ifdef CONFIG_32BIT
> /*
> * Switch to kernel page tables. A full fence is necessary in order to
> * avoid using the trampoline translations, which are only correct for
> @@ -130,6 +138,7 @@ relocate:
> */
> csrw CSR_SATP, a2
> sfence.vma
> +#endif /* CONFIG_32BIT */
>

I would have done something like that instead, in order to avoid to copy
the code that loads the kernel page table:


diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 89cc58ab52b4..1251f4d201a8 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -98,18 +98,23 @@ relocate:
li a1, SATP_MODE
or a2, a2, a1

+ /*
+ * 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
/*
* 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


> ret
> #endif /* CONFIG_MMU */
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 11b61bea0c4d..b7226ac2d04f 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -210,13 +210,17 @@ unsigned long pfn_base __ro_after_init;
> EXPORT_SYMBOL(pfn_base);
>
> pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
> +#ifdef CONFIG_32BIT
> pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
> +#endif /* CONFIG_32BIT */
> 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
> +#ifdef CONFIG_32BIT
> #define trampoline_pg_dir ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
> +#endif /* CONFIG_32BIT */
> #define fixmap_pte ((pte_t *)XIP_FIXUP(fixmap_pte))
> #define early_pg_dir ((pgd_t *)XIP_FIXUP(early_pg_dir))
> #endif /* CONFIG_XIP_KERNEL */
> @@ -291,13 +295,11 @@ static void __init create_pte_mapping(pte_t *ptep,
>
> #ifndef __PAGETABLE_PMD_FOLDED
>
> -pmd_t trampoline_pmd[PTRS_PER_PMD] __page_aligned_bss;
> pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
> pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
> 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 */
> @@ -543,21 +545,12 @@ 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_virt_addr,
> - (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
> -#ifdef CONFIG_XIP_KERNEL
> - create_pmd_mapping(trampoline_pmd, kernel_virt_addr,
> - xiprom, PMD_SIZE, PAGE_KERNEL_EXEC);
> -#else
> - create_pmd_mapping(trampoline_pmd, kernel_virt_addr,
> - load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
> -#endif
> -#else
> +#endif /* __PAGETABLE_PMD_FOLDED */
> +#ifdef CONFIG_32BIT

Why did you change the #else for an #ifdef CONFIG_32BIT?
__PAGETABLE_PMD_FOLDED <=> CONFIG_32BIT anyway.

> /* Setup trampoline PGD */
> create_pgd_mapping(trampoline_pg_dir, kernel_virt_addr,
> load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
> -#endif
> +#endif /* CONFIG_32BIT */

If you use IS_ENABLED(CONFIG_32BIT) instead of #ifdef (and maybe rather
CONFIG_64BIT as this is the config that is widely used to distinguish
between both), that would avoid the #ifdef around trampoline_pg_dir
which would be even more readable.

>
> /*
> * Setup early PGD covering entire kernel which will allow
>

Thanks,

Alex

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