linux-kernel - Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode availability

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Message-ID: <c9c4ca95-0423-a50f-f417-776ccc7904e7@netscape.net>
Date:   Thu, 19 May 2022 22:21:42 -0400
From:   Chuck Zmudzinski <brchuckz@...scape.net>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     jbeulich@...e.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        pkg-xen-devel@...oth-lists.debian.net
Subject: Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode
 availability

On 5/19/22 10:15 PM, Chuck Zmudzinski wrote:
> On 5/3/22 9:22 AM, Juergen Gross wrote:
>> Some drivers are using pat_enabled() in order to test availability of
>> special caching modes (WC and UC-). This will lead to false negatives
>> in case the system was booted e.g. with the "nopat" variant and the
>> BIOS did setup the PAT MSR supporting the queried mode, or if the
>> system is running as a Xen PV guest.
> Hello,
>
> I am also getting a false positive

Sorry, I meant false negative here, not false
positive.

Chuck

> in a Xen Dom0 from
> pat_enabled() where bdd8b6c98239 patched the file
>
> drivers/gpu/drm/i915/gem/i915_gem_pages.c
>
> I think this patch also needs to touch that file to
> fix the issue I am seeing.
...