Re: [PATCH v2 1/4] KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES on nested vmexit

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 07, 2021, Paolo Bonzini wrote:
> On 07/01/21 18:00, Sean Christopherson wrote:
> > Ugh, I assume this is due to one of the "premature" nested_ops->check_events()
> > calls that are necessitated by the event mess?  I'm guessing kvm_vcpu_running()
> > is the culprit?
> > 
> > If my assumption is correct, this bug affects nVMX as well.
> 
> Yes, though it may be latent.  For SVM it was until we started allocating
> svm->nested on demand.
> 
> > Rather than clear the request blindly on any nested VM-Exit, what
> > about something like the following?
> 
> I think your patch is overkill, KVM_REQ_GET_NESTED_STATE_PAGES is only set
> from KVM_SET_NESTED_STATE so it cannot happen while the VM runs.

Yeah, which is why I was hoping we could avoid clearing the request on every
nested exit.

> Something like this is small enough and works well.

I've no argument against it working, rather that I dislike clearing the request
on every exit.  Except for the ->check_events() case, hitting the scenario where
there's a pending request at the time of nested VM-Exit would ideally be treated
as a KVM bug.

On the other hand, clearing nested-specific request on nested VM-Exit is
logically sound, so I guess I'm ok with the minimal patch.