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