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SubjectRe: [PATCH] KVM: nSVM: prepare guest save area while is_guest_mode is true
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On 18/02/21 18:42, Sean Christopherson wrote:
>> The bug is present since commit 06fc7772690d ("KVM: SVM: Activate nested
>> state only when guest state is complete", 2010-04-25). Unfortunately,
>> it is not clear from the commit message what issue exactly led to the
>> change back then. It was probably related to svm_set_cr0 however because
>> the patch series cover letter[1] mentioned lazy FPU switching.
>
> Aha! It was indeed related to svm_set_cr0(). Specifically, the next patch,
> commit 66a562f7e257 ("KVM: SVM: Make lazy FPU switching work with nested svm"),
> added is_nested() checks in update_cr0_intercept() to merge L1's intercepts with
> L0's intercepts.

Yeah, the problem is I don't understand why 06fc7772690d fixed things in
11 year old KVM instead of breaking them, because effectively this patch
is reverting it.

I don't care _that_ much because so much has changed since then; the
world switch logic is abstracted better nowadays, and it is easier to
review the change. But it is weird, nevertheless.

Paolo

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