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


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.

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

Thanks, Jan

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