| Message ID | 20240819130007.33088-1-vkuznets@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | hyperv: Make sure SynIC state is really updated before KVM_RUN | expand |
diff --git a/target/i386/kvm/hyperv.c b/target/i386/kvm/hyperv.c index b94f12acc2c9..70b89cacf94b 100644 --- a/target/i386/kvm/hyperv.c +++ b/target/i386/kvm/hyperv.c @@ -80,6 +80,7 @@ int kvm_hv_handle_exit(X86CPU *cpu, struct kvm_hyperv_exit *exit) * necessary because memory hierarchy is being changed */ async_safe_run_on_cpu(CPU(cpu), async_synic_update, RUN_ON_CPU_NULL); + cpu_exit(CPU(cpu)); return EXCP_INTERRUPT; case KVM_EXIT_HYPERV_HCALL: {
'hyperv_synic' test from KVM unittests was observed to be flaky on certain hardware (hangs sometimes). Debugging shows that the problem happens in hyperv_sint_route_new() when the test tries to set up a new SynIC route. The function bails out on: if (!synic->sctl_enabled) { goto cleanup; } but the test writes to HV_X64_MSR_SCONTROL just before it starts establishing SINT routes. Further investigation shows that synic_update() (called from async_synic_update()) happens after the SINT setup attempt and not before. Apparently, the comment before async_safe_run_on_cpu() in kvm_hv_handle_exit() does not correctly describe the guarantees async_safe_run_on_cpu() gives. In particular, async worked added to a CPU is actually processed from qemu_wait_io_event() which is not always called before KVM_RUN, i.e. kvm_cpu_exec() checks whether an exit request is pending for a CPU and if not, keeps running the vCPU until it meets an exit it can't handle internally. Hyper-V specific MSR writes are not automatically trigger an exit. Fix the issue by simply raising an exit request for the vCPU where SynIC update was queued. This is not a performance critical path as SynIC state does not get updated so often (and async_safe_run_on_cpu() is a big hammer anyways). Reported-by: Jan Richter <jarichte@redhat.com> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com> --- target/i386/kvm/hyperv.c | 1 + 1 file changed, 1 insertion(+)