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 07:45:34 -0400
From:   Chuck Zmudzinski <brchuckz@...scape.net>
To:     Jan Beulich <jbeulich@...e.com>
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>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT
 without MTRR

On 7/18/2022 7:39 AM, Jan Beulich wrote:
> On 18.07.2022 13:31, Chuck Zmudzinski wrote:
> > On 7/18/2022 2:07 AM, Jan Beulich wrote:
> >> 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.
> > 
> > I am not so sure about that, given what Borislav Petkov
> > said when commenting on your patch here:
> > 
> > https://lore.kernel.org/lkml/YsRjX%2FU1XN8rq+8u@zn.tnic/
> > 
> > Specifically, Borislav Petkov wrote on Tue, 5 Jul 2022 18:14:23 +0200:
> > 
> > Actually, the current goal is to adjust Xen dom0 because:
> > 
> > 1. it uses the PAT code
> > 
> > 2. but then it does something special and hides the MTRRs
> > 
> > which is not something real hardware does.
> > 
> > So this one-off thing should be prominent, visible and not get in the
> > way.
> > 
> > --------------end of Borislav Petkov quote-----------
>
> And then, a day later, he said
>
> "So I'm being told that it would be generally beneficial for all kinds of
>  virtualization solutions to be able to support PAT only, without MTRRs
>  so it would be interesting to see how ugly it would become to decouple
>  PAT from MTRRs in Linux..."
>
> > Jan, can you explain this comment by Borislav Petkov about
> > Xen being a "one-off thing" that hides MTRRs and needs
> > to be "adjusted" so it does "not get in the way"?
>
> I'm afraid this isn't the first time that you ask people to explain
> what somebody else said. I don't follow why you think I could better
> explain what Boris said and why than he could do himself.

That is why I also asked Boris to say something now to clarify his
opinion on these matters. Let's wait and see if Boris says something
to clarify his opinion.

Chuck

>
> Jan