linux-kernel - Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when running on e.g. Xen

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Message-ID: <Ysv+Gh0Gk4+uGwrb@zn.tnic>
Date:   Mon, 11 Jul 2022 12:40:26 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] x86/PAT: have pat_enabled() properly reflect state when
 running on e.g. Xen

On Thu, Jul 07, 2022 at 08:38:44AM +0200, Jan Beulich wrote:
> Well, right now the pvops hook for Xen swallows #GP anyway (wrongly
> so imo, but any of my earlier pointing out of that has been left
> unheard, despite even the code comments there saying "It may be worth
> changing that").

Oh great. ;-\

> The point is therefore that after writing PAT, it would need reading
> back. In which case it feels (slightly) more clean to me to avoid the
> write attempt in the first place, when we know it's not going to work.

X86_FEATURE_XENPV check then.

> If I may ask - doesn't this mean this patch, in its current shape, is
> already a (small) step in that direction? In any event what you say
> doesn't sound to me like a viable (backportable) route to addressing
> the regression at hand.

Backportable to where? To whatever tree has

bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")

? If it is that, then 5.17 and newer.

Anyway, I don't mind it as long as you put the proper splitting out
ontop and it all goes as a single patchset, with the minimal fix
CC:stable and queued first.

Thx.

-- 
Regards/Gruss,
    Boris.

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