QEMU's Hyper-V HV_X64_MSR_EOM is broken with split IRQCHIP

[Sean Christopherson <seanjc@xxxxxxxxxx>]

FYI, QEMU's Hyper-V emulation of HV_X64_MSR_EOM has been broken since QEMU commit
c82d9d43ed ("KVM: Kick resamplefd for split kernel irqchip"), as nothing in KVM
will forward the EOM notification to userspace.  I have no idea if anything in
QEMU besides hyperv_testdev.c cares.

The bug is reproducible by running the hyperv_connections KVM-Unit-Test with a
split IRQCHIP.

Hacking QEMU and KVM (see KVM commit 654f1f13ea56 ("kvm: Check irqchip mode before
assign irqfd") as below gets the test to pass.  Assuming that's not a palatable
solution, the other options I can think of would be for QEMU to intercept
HV_X64_MSR_EOM when using a split IRQCHIP, or to modify KVM to do KVM_EXIT_HYPERV_SYNIC
on writes to HV_X64_MSR_EOM with a split IRQCHIP.

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index c65b790433..820bc1692e 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2261,10 +2261,9 @@ static int kvm_irqchip_assign_irqfd(KVMState *s, EventNotifier *event,
              * the INTx slow path).
              */
             kvm_resample_fd_insert(virq, resample);
-        } else {
-            irqfd.flags |= KVM_IRQFD_FLAG_RESAMPLE;
-            irqfd.resamplefd = rfd;
         }
+        irqfd.flags |= KVM_IRQFD_FLAG_RESAMPLE;
+        irqfd.resamplefd = rfd;
     } else if (!assign) {
         if (kvm_irqchip_is_split()) {
             kvm_resample_fd_remove(virq);


diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index 63f66c51975a..0bf85f89eb27 100644
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -166,9 +166,7 @@ void __kvm_migrate_timers(struct kvm_vcpu *vcpu)
 
 bool kvm_arch_irqfd_allowed(struct kvm *kvm, struct kvm_irqfd *args)
 {
-       bool resample = args->flags & KVM_IRQFD_FLAG_RESAMPLE;
-
-       return resample ? irqchip_kernel(kvm) : irqchip_in_kernel(kvm);
+       return irqchip_in_kernel(kvm);
 }
 
 bool kvm_arch_irqchip_in_kernel(struct kvm *kvm)