From: donsheng <dongsheng.x.zh...@intel.com> This bug fix addresses the incorrect return value of kvm_hv_handle_exit for KVM_EXIT_HYPERV_SYNIC, which should be EXCP_INTERRUPT.
Handling of KVM_EXIT_HYPERV_SYNIC in QEMU needs to be synchronous. This means that async_synic_update should run in the current QEMU vCPU thread before returning to KVM, returning EXCP_INTERRUPT to guarantee this. Returning 0 can cause async_synic_update to run asynchronously. One problem (kvm-unit-tests's hyperv_synic test fails with timeout error) caused by this bug: When a guest VM writes to the HV_X64_MSR_SCONTROL MSR to enable Hyper-V SynIC, a VM exit is triggered and processed by the kvm_hv_handle_exit function of the QEMU vCPU. This function then calls the async_synic_update function to set synic->sctl_enabled to true. A true value of synic->sctl_enabled is required before creating SINT routes using the hyperv_sint_route_new() function. If kvm_hv_handle_exit returns 0 for KVM_EXIT_HYPERV_SYNIC, the current QEMU vCPU thread may return to KVM and enter the guest VM before running async_synic_update. In such case, the hyperv_synic test’s subsequent call to synic_ctl(HV_TEST_DEV_SINT_ROUTE_CREATE, ...) immediately after writing to HV_X64_MSR_SCONTROL can cause QEMU’s hyperv_sint_route_new() function to return prematurely (because synic->sctl_enabled is false). If the SINT route is not created successfully, the SINT interrupt will not be fired, resulting in a timeout error in the hyperv_synic test. Fixes: 267e071bd6d6 (“hyperv: make overlay pages for SynIC”) Suggested-by: Chao Gao <chao....@intel.com> Signed-off-by: Dongsheng Zhang <dongsheng.x.zh...@intel.com> --- target/i386/kvm/hyperv.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/target/i386/kvm/hyperv.c b/target/i386/kvm/hyperv.c index f2a3fe650a18..b94f12acc2c9 100644 --- a/target/i386/kvm/hyperv.c +++ b/target/i386/kvm/hyperv.c @@ -81,7 +81,7 @@ int kvm_hv_handle_exit(X86CPU *cpu, struct kvm_hyperv_exit *exit) */ async_safe_run_on_cpu(CPU(cpu), async_synic_update, RUN_ON_CPU_NULL); - return 0; + return EXCP_INTERRUPT; case KVM_EXIT_HYPERV_HCALL: { uint16_t code = exit->u.hcall.input & 0xffff; bool fast = exit->u.hcall.input & HV_HYPERCALL_FAST; -- 2.43.0