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

Sean Christopherson
2022-02-10T23:16:27Z

On Thu, Feb 10, 2022, Sean Christopherson wrote:
> 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.

Third time's a charm...

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_paging(vcpu);