Messages in this thread | | | Subject | Re: Re: [RFC] KVM: x86: SVM: don't expose PV_SEND_IPI feature with AVIC | From | zhenwei pi <> | Date | Mon, 8 Nov 2021 19:14:49 +0800 |
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On 11/8/21 7:08 PM, Maxim Levitsky wrote: > On Mon, 2021-11-08 at 11:30 +0100, Paolo Bonzini wrote: >> On 11/8/21 10:59, Kele Huang wrote: >>> 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@caviumnetworks.com/ >>> 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@bytedance.com> >> >> AVIC can change across migration. I think we should instead use a new >> KVM_HINTS_* bit (KVM_HINTS_ACCELERATED_LAPIC or something like that). >> The KVM_HINTS_* bits are intended to be changeable across migration, >> even though we don't have for now anything equivalent to the Hyper-V >> reenlightenment interrupt. > > Note that the same issue exists with HyperV. It also has PV APIC, > which is harmful when AVIC is enabled (that is guest uses it instead > of using AVIC, negating AVIC benefits). > > Also note that Intel recently posted IPI virtualizaion, which > will make this issue relevant to APICv too soon. > > I don't yet know if there is a solution to this which doesn't > involve some management software decision (e.g libvirt or higher). > > Best regards, > Maxim Levitsky >
For QEMU, "-cpu host,kvm-pv-ipi=off" can disable kvm-pv-ipi. And for libvirt, I posted a patch to disable kvm-pv-ipi by libvirt xml, link: https://github.com/libvirt/libvirt/commit/b2757b697e29fa86972a4638a5879dccc8add2ad
>> >> Paolo >> >>> --- >>> 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); >>> } >>> > >
-- zhenwei pi
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