linux-kernel - Re: [PATCH] arm64: errata: refer to config ARM64_ERRATUM

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Date:   Thu, 15 Dec 2022 10:57:46 +0000
From:   Will Deacon <will@...nel.org>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: errata: refer to config ARM64_ERRATUM_2645198 to
 make workaround work

On Thu, Dec 15, 2022 at 10:48:11AM +0100, Lukas Bulwahn wrote:
> Commit 44ecda71fd8a ("arm64: errata: Workaround possible Cortex-A715
> [ESR|FAR]_ELx corruption") implements a workaround for arm64 erratum
> 2645198. The arm64 cpucaps is called WORKAROUND_2645198; the kernel build
> configuration is called ARM64_ERRATUM_2645198.
> 
> In the functions huge_ptep_modify_prot_start() and
> ptep_modify_prot_start(), the code accidently refers to the non-existing
> config CONFIG_ARM64_WORKAROUND_2645198. Note that the config name uses
> ERRATUM, not WORKAROUND. By this accidental misreference, this condition is
> always false, the branch of the workaround is not reachable and the
> workaround is effectively not implemented at all.
> 
> Refer to the intended config ARM64_ERRATUM_2645198 and make the intended
> workaround effectively work.
> 
> Fixes: 44ecda71fd8a ("arm64: errata: Workaround possible Cortex-A715 [ESR|FAR]_ELx corruption")
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
> ---
>  arch/arm64/mm/hugetlbpage.c | 2 +-
>  arch/arm64/mm/mmu.c         | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)

Thanks for the report!

> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index cd8d96e1fa1a..95364e8bdc19 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -562,7 +562,7 @@ bool __init arch_hugetlb_valid_size(unsigned long size)
>  
>  pte_t huge_ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
>  {
> -	if (IS_ENABLED(CONFIG_ARM64_WORKAROUND_2645198) &&
> +	if (IS_ENABLED(CONFIG_ARM64_ERRATUM_2645198) &&
>  	    cpus_have_const_cap(ARM64_WORKAROUND_2645198)) {
>  		/*
>  		 * Break-before-make (BBM) is required for all user space mappings
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 12915f379c22..d77c9f56b7b4 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1633,7 +1633,7 @@ early_initcall(prevent_bootmem_remove_init);
>  
>  pte_t ptep_modify_prot_start(struct vm_area_struct *vma, unsigned long addr, pte_t *ptep)
>  {
> -	if (IS_ENABLED(CONFIG_ARM64_WORKAROUND_2645198) &&
> +	if (IS_ENABLED(CONFIG_ARM64_ERRATUM_2645198) &&
>  	    cpus_have_const_cap(ARM64_WORKAROUND_2645198)) {
>  		/*
>  		 * Break-before-make (BBM) is required for all user space mappings

Grr, this bug seems to exist in all three versions of the patch reviewed on
the list, so I can only draw the conclusion that this code has never been
tested. Consequently, I'm more inclined to _revert_ the change for now and
we can bring it back as a fix once somebody has checked that it actually
works properly.

Will