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

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Date:   Mon, 20 Jun 2022 07:30:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Thorsten Leemhuis <regressions@...mhuis.info>,
        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 20.06.22 07:22, Thorsten Leemhuis wrote:
> 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/

No, patch 2 wasn't covering all needed cases.

> Is Patch 1 alone enough to fix it? Or is there a different fix for it?

Patch 1 is fixing a different issue (it is lacking any maintainer
feedback, though).

This patch of Jan should do the job, but it seems to be stuck, too:

https://lore.kernel.org/lkml/9385fa60-fa5d-f559-a137-6608408f88b0@suse.com/

> Or is there some other solution to finally fix that regressions that
> ideally should have been fixed weeks ago already?

I agree it should have been fixed quite some time now, but the x86
maintainers don't seem to be interested in those stuck patches. :-(

Maybe I should take a different approach:

x86 maintainers, please speak up if you NAK (or Ack) any of above two patches.
In case you don't NAK or take the patches, I'm inclined to carry them via
the Xen tree to get the issues fixed.


Juergen

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