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

Sean Christopherson
2022-02-10T23:11:05Z

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;
}

> return false;
>
> if (!kvm_pv_async_pf_enabled(vcpu) ||
> --
> 2.31.1
>
>