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    SubjectRe: [PATCH] KVM: nSVM: prepare guest save area while is_guest_mode is true
    On Thu, Feb 18, 2021, Paolo Bonzini wrote:
    > 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.

    11 year old KVM didn't grab a different VMCB when updating the intercepts, it
    had already copied/merged L1's stuff to L0's VMCB, and then updated L0's VMCB
    regardless of is_nested().

    > 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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