linux-kernel - Re: [PATCH v2 -next] riscv: mm: remove redundant trampoline PGD for 64bit

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Message-ID: <3199c658-ffac-729d-d93d-8895e7d3668e@ghiti.fr>
Date:   Wed, 28 Jul 2021 13:55:01 +0200
From:   Alex Ghiti <alex@...ti.fr>
To:     Nanyong Sun <sunnanyong@...wei.com>, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu, anup.patel@....com
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        palmerdabbelt@...gle.com, atish.patra@....com,
        wangkefeng.wang@...wei.com, tiantao6@...ilicon.com,
        qiuwenbo@...tium.com.cn, rppt@...nel.org, jszhang@...nel.org,
        mick@....forth.gr
Subject: Re: [PATCH v2 -next] riscv: mm: remove redundant trampoline PGD for
 64bit



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/20190325092234.5451-4-anup.patel@wdc.com/
> [2]https://lkml.org/lkml/2019/3/28/147
> 
> Signed-off-by: Nanyong Sun <sunnanyong@...wei.com>
> Reviewed-by: Alexandre Ghiti <alex@...ti.fr>

I have not reviewed this patch yet! :)

> Reviewed-by: Kefeng Wang <wangkefeng.wang@...wei.com>
> Reviewed-by: weiyongjun <weiyongjun1@...wei.com>
> ---
> 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,
>