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

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Date:   Wed, 4 May 2022 11:51:17 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     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>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode
 availability

On 04.05.2022 11:14, Juergen Gross wrote:
> On 04.05.22 10:31, Jan Beulich wrote:
>> On 03.05.2022 15:22, 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.
>>
>> While, as per my earlier patch, I agree with the Xen PV case, I'm not
>> convinced "nopat" is supposed to honor firmware-provided settings. In
>> fact in my patch I did arrange for "nopat" to also take effect under
>> Xen PV.
> 
> Depends on what the wanted semantics for "nopat" are.
> 
> Right now "nopat" will result in the PAT MSR left unchanged and the
> cache mode translation tables be initialized accordingly.
> 
> So does "nopat" mean that the PAT MSR shouldn't be changed, or that
> PAGE_BIT_PAT will never be set?

According to the documentation for the option ("Disable PAT (page
attribute table extension of pagetables) support") I'd say the latter.

Jan