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:
- 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).
- 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