KVM: How does is PAT emulation supposed to work?

[Jan Kiszka <jan.kiszka@siemens.com>]

Hi all,

while digging into the PAT topic for Jailhouse, I also wondered how KVM
deals with it. And I'm still not getting it complete - or there is a bug:

KVM intercepts all guest writes to the PAT MSR and instead keeps the
guest value in vcpu->arch.pat. But, besides returning that value back on
read accesses, arch.pat has no other purpose.

On Intel, we only seem to have proper emulation - through hardware -
when VMX supports PAT switching (see vmx_set_msr). On AMD, the situation
is even worse as the g_pat save field is not updated at all on PAT
writes. That seems to be a low hanging fruit to bring svm on the same
support level as vmx.

Or am I missing something?

Jan

PS: If someone has a good idea for a simple test case on machines
without IOMMU (like my current boxes), thus without a chance to use
device pass-through to stress guest PAT settings, I would be all ears.

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