Currently, AVIC is disabled if x2apic feature is exposed to guest or in-kernel PIT is in re-injection mode. We can enable AVIC with options: Kmod args: modprobe kvm_amd avic=1 nested=0 npt=1 QEMU args: ... -cpu host,-x2apic -global kvm-pit.lost_tick_policy=discard ... When LAPIC works in xapic mode, both AVIC and PV_SEND_IPI feature can accelerate IPI operations for guest. However, the relationship between AVIC and PV_SEND_IPI feature is not sorted out. In logical, AVIC accelerates most of frequently IPI operations without VMM intervention, while the re-hooking of apic->send_IPI_xxx from PV_SEND_IPI feature masks out it. People can get confused if AVIC is enabled while getting lots of hypercall kvm_exits from IPI. In performance, benchmark tool https://lore.kernel.org/kvm/20171219085010.4081-1-ynorov@xxxxxxxxxxxxxxxxxx/ shows below results: Test env: CPU: AMD EPYC 7742 64-Core Processor 2 vCPUs pinned 1:1 idle=poll Test result (average ns per IPI of lots of running): PV_SEND_IPI : 1860 AVIC : 1390 Besides, disscussions in https://lkml.org/lkml/2021/10/20/423 do have some solid performance test results to this. This patch fixes this by masking out PV_SEND_IPI feature when AVIC is enabled in setting up of guest vCPUs' CPUID. Signed-off-by: Kele Huang <huangkele@xxxxxxxxxxxxx> --- arch/x86/kvm/cpuid.c | 4 ++-- arch/x86/kvm/svm/svm.c | 13 +++++++++++++ 2 files changed, 15 insertions(+), 2 deletions(-) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 2d70edb0f323..cc22975e2ac5 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -194,8 +194,6 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) best->ecx |= XFEATURE_MASK_FPSSE; } - kvm_update_pv_runtime(vcpu); - vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu); vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu); @@ -208,6 +206,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) /* Invoke the vendor callback only after the above state is updated. */ static_call(kvm_x86_vcpu_after_set_cpuid)(vcpu); + kvm_update_pv_runtime(vcpu); + /* * Except for the MMU, which needs to do its thing any vendor specific * adjustments to the reserved GPA bits. diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index b36ca4e476c2..b13bcfb2617c 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -4114,6 +4114,19 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu) if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM)) kvm_request_apicv_update(vcpu->kvm, false, APICV_INHIBIT_REASON_NESTED); + + if (!guest_cpuid_has(vcpu, X86_FEATURE_X2APIC) && + !(nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))) { + /* + * PV_SEND_IPI feature masks out AVIC acceleration to IPI. + * So, we do not expose PV_SEND_IPI feature to guest when + * AVIC is enabled. + */ + best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0); + if (best && enable_apicv && + (best->eax & (1 << KVM_FEATURE_PV_SEND_IPI))) + best->eax &= ~(1 << KVM_FEATURE_PV_SEND_IPI); + } } init_vmcb_after_set_cpuid(vcpu); } -- 2.11.0
