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

[Chuck Zmudzinski <brchuckz@netscape.net>]

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-----------

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"?

Borislav, are you willing to retract the comments you made
on Tue, 5 Jul 2022 18:14:23 +0200 about Xen?

>
> Jan

Jan, thanks for answering my questions.

Chuck