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

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Message-ID: <20230602144357.GCZHoALQjO+xx3YxAz@fat_crate.local>
Date:   Fri, 2 Jun 2023 16:43:57 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
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 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?

It says above it "Set of bits not changed in pte_modify."

And I don't see pte_modify() changing that bit either.

Right now this "fix" looks like, "let's OR these two masks so that we
can take care of _PAGE_PAT too". But it doesn't make a whole lotta sense
to me...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette