linux-kernel - Re: [RESUBMIT][PATCH] x86/mm: Fix PAT bit missing from page protection modify mask

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Date:   Fri, 2 Jun 2023 16:53:30 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Janusz Krzysztofik <janusz.krzysztofik@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>, linux-kernel@...r.kernel.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>,
        Andi Shyti <andi.shyti@...ux.intel.com>,
        drm-intel@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [RESUBMIT][PATCH] x86/mm: Fix PAT bit missing from page
 protection modify mask

On 02.06.23 16:48, Juergen Gross wrote:
> On 02.06.23 16:43, Borislav Petkov wrote:
>> On Thu, Jun 01, 2023 at 10:47:39AM +0200, Juergen Gross wrote:
>>> As described in the commit message, this only works on bare metal due to the
>>> PAT bit not being needed for WC mappings.
>>>
>>> Making this patch Xen specific would try to cure the symptoms without fixing
>>> the underlying problem: _PAGE_PAT should be regarded the same way as the bits
>>> for caching mode (_PAGE_CHG_MASK).
>>
>> So why isn't _PAGE_PAT part of _PAGE_CHG_MASK?
> 
> This would result in problems for large pages: _PAGE_PSE is at the same
> position as _PAGE_PAT (large pages are using _PAGE_PAT_LARGE instead).
> 
> Yes, x86 ABI is a mess.

Oh, wait: I originally thought _PAGE_CHG_MASK would be used for large pages,
too. There is _HPAGE_CHG_MASK for that purpose.

So adding _PAGE_PAT to _PAGE_CHG_MASK and _PAGE_PAT_LARGE to _HPAGE_CHG_MASK
should do the job. At least I hope so.


Juergen

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