Re: [PATCH 03/12] KVM: x86: do not deliver asynchronous page faults if CR0.PG=0

Sean Christopherson
2022-02-10T23:14:27Z

On Thu, Feb 10, 2022, Sean Christopherson wrote:
> On Wed, Feb 09, 2022, Paolo Bonzini wrote:
> > Enabling async page faults is nonsensical if paging is disabled, but
> > it is allowed because CR0.PG=0 does not clear the async page fault
> > MSR. Just ignore them and only use the artificial halt state,
> > similar to what happens in guest mode if async #PF vmexits are disabled.
> >
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> > arch/x86/kvm/x86.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5e1298aef9e2..98aca0f2af12 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12272,7 +12272,9 @@ static inline bool apf_pageready_slot_free(struct kvm_vcpu *vcpu)
> >
> > static bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
> > {
> > - if (!vcpu->arch.apf.delivery_as_pf_vmexit && is_guest_mode(vcpu))
> > + if (is_guest_mode(vcpu)
> > + ? !vcpu->arch.apf.delivery_as_pf_vmexit
> > + : !is_cr0_pg(vcpu->arch.mmu))
>
> As suggested in the previous patch, is_paging(vcpu).
>
> I find a more tradition if-elif marginally easier to understand the implication
> that CR0.PG is L2's CR0 and thus irrelevant if is_guest_mode()==true. Not a big
> deal though.
>
> if (is_guest_mode(vcpu)) {
> if (!vcpu->arch.apf.delivery_as_pf_vmexit)
> return false;
> } else if (!is_paging(vcpu)) {
> return false;
> }

Alternatively, what about reordering and refactoring to yield:

if (kvm_pv_async_pf_enabled(vcpu))
return false;

if (vcpu->arch.apf.send_user_only &&
static_call(kvm_x86_get_cpl)(vcpu) == 0)
return false;

/* L1 CR0.PG=1 is guaranteed if the vCPU is in guest mode (L2). */
if (is_guest_mode(vcpu)
return !vcpu->arch.apf.delivery_as_pf_vmexit;

return is_cr0_pg(vcpu->arch.mmu);

There isn't any need to "batch" the if statements.