From 327e8c65d28dc357c02b508e6485e7c57d4d1efa Mon Sep 17 00:00:00 2001 From: Vitaly Kuznetsov Date: Tue, 7 Jan 2025 13:43:32 +0100 Subject: [PATCH 1/4] target/i386: Make sure SynIC state is really updated before KVM_RUN RH-Author: Vitaly Kuznetsov RH-MergeRequest: 314: target/i386: Make sure SynIC state is really updated before KVM_RUN RH-Jira: RHEL-73002 RH-Acked-by: Paolo Bonzini RH-Acked-by: Maxim Levitsky RH-Commit: [1/1] 2d8f7605e3efd3c76d16a2cb9e7c0898786fb4e9 (vkuznets/qemu-kvm) '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 Signed-off-by: Vitaly Kuznetsov Link: https://lore.kernel.org/r/20240917160051.2637594-4-vkuznets@redhat.com Signed-off-by: Paolo Bonzini (cherry picked from commit d3177e2e4353824a650434c57471615d43507500) Signed-off-by: Vitaly Kuznetsov --- target/i386/kvm/hyperv.c | 1 + 1 file changed, 1 insertion(+) diff --git a/target/i386/kvm/hyperv.c b/target/i386/kvm/hyperv.c index b94f12acc2..70b89cacf9 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: { -- 2.39.3