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

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 04, 2025, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> 
> > 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 only VMBus device in QEMU besides the testdev seems to be Hyper-V
> ballooning driver, Cc: Maciej to check whether it's a real problem for
> it or not.
> 
> >
> > The bug is reproducible by running the hyperv_connections KVM-Unit-Test with a
> > split IRQCHIP.
> 
> Thanks, I can reproduce the problem too.
> 
> >
> > 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.
> 
> AFAIR, Hyper-V message interface is a fairly generic communication
> mechanism which in theory can be used without interrupts at all: the
> corresponding SINT can be masked and the guest can be polling for
> messages, proccessing them and then writing to HV_X64_MSR_EOM to trigger
> delivery on the next queued message. To support this scenario on the
> backend, we need to receive HV_X64_MSR_EOM writes regardless of whether
> irqchip is split or not. (In theory, we can get away without this by
> just checking if pending messages can be delivered upon each vCPU entry
> but this can take an undefined amount of time in some scenarios so I
> guess we're better off with notifications).

Before c82d9d43ed ("KVM: Kick resamplefd for split kernel irqchip"), and without
a split IRCHIP, QEMU gets notified via eventfd.  On writes to HV_X64_MSR_EOM, KVM
invokes irq_acked(), i.e. irqfd_resampler_ack(), for all SINT routes.  The eventfd
signal gets back to sint_ack_handler(), which invokes msg_retry() to re-post the
message.

I.e. trapping HV_X64_MSR_EOM on would be a slow path relative to what's there for
in-kernel IRQCHIP.