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

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Date:   Thu, 15 Dec 2022 19:44:14 +0000
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        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 Thu, Dec 15, 2022 at 04:59:20PM +0530, Anshuman Khandual wrote:
> On 12/15/22 16:27, Will Deacon wrote:
> > On Thu, Dec 15, 2022 at 10:48:11AM +0100, Lukas Bulwahn wrote:
> >> 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).

"might have"?

> > 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.

I've gone ahead with the revert anyway, just because it's the easy thing to
do and we can bring back a fixed version of the patch as a fix in the new
year. So please send a new version with this fix folded in after you've
tested that it doesn't cause regressions for systems without the erratum.

Cheers,

Will