linux-kernel - Re: [PATCH 1/2] x86/pat: fix x86_has_pat

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Date:   Mon, 20 Jun 2022 07:22:18 +0200
From:   Thorsten Leemhuis <regressions@...mhuis.info>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     jbeulich@...e.com, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/2] x86/pat: fix x86_has_pat_wp()

On 14.06.22 17:09, Juergen Gross wrote:
> On 03.05.22 15:22, Juergen Gross wrote:
>> x86_has_pat_wp() is using a wrong test, as it relies on the normal
>> PAT configuration used by the kernel. In case the PAT MSR has been
>> setup by another entity (e.g. BIOS or Xen hypervisor) it might return
>> false even if the PAT configuration is allowing WP mappings.
>>
>> Fixes: 1f6f655e01ad ("x86/mm: Add a x86_has_pat_wp() helper")
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>>   arch/x86/mm/init.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
>> index d8cfce221275..71e182ebced3 100644
>> --- a/arch/x86/mm/init.c
>> +++ b/arch/x86/mm/init.c
>> @@ -80,7 +80,8 @@ static uint8_t __pte2cachemode_tbl[8] = {
>>   /* Check that the write-protect PAT entry is set for write-protect */
>>   bool x86_has_pat_wp(void)
>>   {
>> -    return __pte2cachemode_tbl[_PAGE_CACHE_MODE_WP] ==
>> _PAGE_CACHE_MODE_WP;
>> +    return
>> __pte2cachemode_tbl[__cachemode2pte_tbl[_PAGE_CACHE_MODE_WP]] ==
>> +           _PAGE_CACHE_MODE_WP;
>>   }
>>     enum page_cache_mode pgprot2cachemode(pgprot_t pgprot)
> 
> x86 maintainers, please consider taking this patch, as it is fixing
> a real bug. Patch 2 of this series can be dropped IMO.

Juergen, can you help me out here please. Patch 2 afaics was supposed to
fix this regression I'm tracking:
https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/

Is Patch 1 alone enough to fix it? Or is there a different fix for it?
Or is there some other solution to finally fix that regressions that
ideally should have been fixed weeks ago already?

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)

P.S.: As the Linux kernel's regression tracker I deal with a lot of
reports and sometimes miss something important when writing mails like
this. If that's the case here, don't hesitate to tell me in a public
reply, it's in everyone's interest to set the public record straight.