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@xxxxxxx/ 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
