Re: [PATCH] KVM: nSVM: prepare guest save area while is_guest_mode is true

[Paolo Bonzini]

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