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

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Date:   Thu, 15 Dec 2022 16:59:20 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Will Deacon <will@...nel.org>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....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 12/15/22 16:27, Will Deacon wrote:
> 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

Ohh, my bad, apologies. I did not have a real system with this erratum, although
had emulated and tested this workaround path via some other debug changes (which
might have just forced the first condition to always evaluate true).

> 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.
Please do not revert this change if possible.