[PATCH v2 02/18] KVM: x86: do not deliver asynchronous page faults if CR0.PG=0

Paolo Bonzini
2022-02-17T21:04:05Z

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.

Given the increasingly complex logic, and the nicer code if the new
"if" is placed last, opportunistically change the "||" into a chain
of "if (...) return false" statements.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/x86.c | 22 ++++++++++++++++++----
1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 99a58c25f5c2..b912eef5dc1a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12270,14 +12270,28 @@ 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 (!kvm_pv_async_pf_enabled(vcpu))
return false;

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

- return true;
+ if (is_guest_mode(vcpu)) {
+ /*
+ * L1 needs to opt into the special #PF vmexits that are
+ * used to deliver async page faults.
+ */
+ return vcpu->arch.apf.delivery_as_pf_vmexit;
+ } else {
+ /*
+ * Play it safe in case the guest does a quick real mode
+ * foray. The real mode IDT is unlikely to have a #PF
+ * exception setup.
+ */
+ return is_paging(vcpu);
+ }
}

bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu)
--
2.31.1