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Re: [Xen-devel] x2APIC MSR range (XSA-108 follow-up)

On Mon, Oct 13, 2014 at 07:45:58AM +0100, Jan Beulich wrote:
> All,
> during the embargo period of XSA-108 Matt pointed out that restricting
> the emulated MSR range to 0x800-0x8ff isn't necessarily the ultimately
> correct thing to do (as also noted in commit 61fdda7acf's description):
> The x2APIC MSR range really is being specified as 0x800-0xbff, as
> opposed to the range considered for virtualization purposes
> (0x800-0x8ff). In order to determine proper behavior here we'd like to
> get clarification from you, particularly also in the light of probing real
> hardware pointing out the existence of (at least) MSRs 0xa00-0xa02.

I recall the MSRs in question being 0xa00 and 0xa01. Perhaps 0xa02
also provided a value, but I'm not as sure.

> With what we currently do (kind of supported by their values at least
> not differing across physical CPUs on the probed systems) their values
> are getting passed through to guests. The alternative of forcing #GP
> for accesses to them as one could imply from the spec seems
> undesirable: Guests may imply their existence based on CPU model.
> Hence the only apparent reasonable alternative would be to provide
> proper virtualization for these registers, requiring to know their
> purpose.

As MSRs that are not publicly documented, I suspect that 0xa00 and
0xa01 don't have semantics that have meaning in the virtualization
context. Confirmation from Intel is welcome so we can determine the
right path.


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