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SubjectRe: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from SMM
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On 23/06/21 18:21, Sean Christopherson wrote:
> On Wed, Jun 23, 2021, Sean Christopherson wrote:
>> And I believe this hackery is necessary only because nested_svm_vmexit() isn't
>> following the architcture in the first place. I.e. using vmcb01 to restore
>> host state is flat out wrong.
>
> Ah, that's not true, using vmcb01 is allowed by "may store some or all host state
> in hidden on-chip memory".

And also, "Different implementations may choose to save the hidden parts
of the host’s segment registers as well as the selectors".

> From a performance perspective, I do like the SMI/RSM shenanigans. I'm not
> totally opposed to the trickery since I think it will break a guest if and only
> if the L1 guest is also violating the APM. And we're not fudging the spec thaat
> much :-)

Yeah, that was my reasoning as well. Any reference to "hidden on-chip
memory", plus the forbidding modifications of the host save area, sort
of implies that the processor can actually flush that hidden on-chip
memory for whatever reason (such as on some sleep states?!?).

Paolo

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