KVM: VMX: Skip Posted Interrupt updates if APICv is hard disabled

Explicitly skip posted interrupt updates if APICv is disabled in all of
KVM, or if the guest doesn't have an in-kernel APIC.  The PI descriptor
is kept up-to-date if APICv is inhibited, e.g. so that re-enabling APICv
doesn't require a bunch of updates, but neither the module param nor the
APIC type can be changed on-the-fly.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 41f946e..ee4ddc6 100644 (file)
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -29,11 +29,14 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
        unsigned int dest;
 
        /*
-        * In case of hot-plug or hot-unplug, we may have to undo
-        * vmx_vcpu_pi_put even if there is no assigned device.  And we
-        * always keep PI.NDST up to date for simplicity: it makes the
-        * code easier, and CPU migration is not a fast path.
+        * To simplify hot-plug and dynamic toggling of APICv, keep PI.NDST and
+        * PI.SN up-to-date even if there is no assigned device or if APICv is
+        * deactivated due to a dynamic inhibit bit, e.g. for Hyper-V's SyncIC.
         */
+       if (!enable_apicv || !lapic_in_kernel(vcpu))
+               return;
+
+       /* Nothing to do if PI.SN and PI.NDST both have the desired value. */
        if (!pi_test_sn(pi_desc) && vcpu->cpu == cpu)
                return;