Re: [PATCH v4 11/11] KVM: x86: First attempt at converting nested virtual APIC page to gpc

[David Woodhouse <dwmw2@infradead.org>]

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On Sat, 2021-11-20 at 18:30 +0200, Mika Penttilä wrote:
> 
> On 20.11.2021 18.21, David Woodhouse wrote:
> > On Sat, 2021-11-20 at 17:48 +0200, Mika Penttilä wrote:
> > > > @@ -9785,6 +9787,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >         local_irq_disable();
> > > >         vcpu->mode = IN_GUEST_MODE;
> > > >     
> > > > +     /*
> > > > +      * If the guest requires direct access to mapped L1 pages, check
> > > > +      * the caches are valid. Will raise KVM_REQ_GET_NESTED_STATE_PAGES
> > > > +      * to go and revalidate them, if necessary.
> > > > +      */
> > > > +     if (is_guest_mode(vcpu) && kvm_x86_ops.nested_ops->check_guest_maps)
> > > > +             kvm_x86_ops.nested_ops->check_guest_maps(vcpu);
> > >
> > > But KVM_REQ_GET_NESTED_STATE_PAGES is not check until next
> > > vcpu_enter_guest() entry ?
> >
> > Sure, but that's why this call to ->check_guest_maps() comes just a few
> > lines *before* the 'if (kvm_cpu_exit_request(vcpu))' that will bounce
> > us back out so that we go through vcpu_enter_guest() from the start
> > again?
>
> Yes, I had forgotten that...=

Having said that, I think Paolo was rightly trying to persuade me that
we don't need the check there at all anyway. Because the *invalidation*
will raise KVM_REQ_GPC_INVALIDATE, and that's sufficient to bounce us
out of IN_GUEST_MODE without us having to check the GPC explicitly.

We do need to ensure that we are up to date with memslots changes, but
that's a bit simpler, and we have kvm_arch_memslots_changed() waking us
for that too.

So yeah, with sufficient thought I think we can be entirely event-
driven instead of having to have an explicit *check* on entry.




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