linux-kernel - Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT without MTRR

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Date:   Mon, 18 Jul 2022 08:07:56 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Chuck Zmudzinski <brchuckz@...scape.net>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Jane Chu <jane.chu@...cle.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org, stable@...r.kernel.org,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT
 without MTRR

On 15.07.2022 21:53, Chuck Zmudzinski wrote:
> Two things I see here in my efforts to get a patch to fix this regression:
> 
> 1. Does Xen have plans to give Linux running in Dom0 write-access to the
> PAT MSR?

No, as this is not technically feasible (all physical CPUs should run
with the same value in the MSR, or else other issues arise).

> 2. Does Xen have plans to expose MTRRs to Linux running in Dom0?

Yen does expose MTRRs to PV Dom0, but via a hypercall mechanism. I
don't think there are plans on the Xen side to support the MSR
interface (and hence to expose the CPUID bit), and iirc there are
no plans on the Linux side to use the MTRR interface. This also
wouldn't really make sense anymore now that it has become quite
clear that Linux wants to have PAT working without depending on
MTRR.

Jan