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SubjectRe: [PATCH v2 5/5] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use
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On 18/05/21 16:43, Vitaly Kuznetsov wrote:
> APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon
> SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is,
> however, possible to track whether the feature was actually used by the
> guest and only inhibit APICv/AVIC when needed.
>
> TLFS suggests a dedicated 'HV_DEPRECATING_AEOI_RECOMMENDED' flag to let
> Windows know that AutoEOI feature should be avoided. While it's up to
> KVM userspace to set the flag, KVM can help a bit by exposing global
> APICv/AVIC enablement: in case APICv/AVIC usage is impossible, AutoEOI
> is still preferred.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Should it also disable APICv unconditionally if
HV_DEPRECATING_AEOI_RECOMMENDED is not in the guest CPUID? That should
avoid ping-pong between enabled and disabled APICv even in pathological
cases that we cannot think about.

Paolo

> ---
> arch/x86/include/asm/kvm_host.h | 3 +++
> arch/x86/kvm/hyperv.c | 27 +++++++++++++++++++++------
> 2 files changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index bf5807d35339..5e03ab4c0e4f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -936,6 +936,9 @@ struct kvm_hv {
> /* How many vCPUs have VP index != vCPU index */
> atomic_t num_mismatched_vp_indexes;
>
> + /* How many SynICs use 'AutoEOI' feature */
> + atomic_t synic_auto_eoi_used;
> +
> struct hv_partition_assist_pg *hv_pa_pg;
> struct kvm_hv_syndbg hv_syndbg;
> };
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index f98370a39936..89e7d5b99279 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -87,6 +87,10 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
> static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
> int vector)
> {
> + struct kvm_vcpu *vcpu = hv_synic_to_vcpu(synic);
> + struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
> + int auto_eoi_old, auto_eoi_new;
> +
> if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
> return;
>
> @@ -95,10 +99,25 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
> else
> __clear_bit(vector, synic->vec_bitmap);
>
> + auto_eoi_old = bitmap_weight(synic->auto_eoi_bitmap, 256);
> +
> if (synic_has_vector_auto_eoi(synic, vector))
> __set_bit(vector, synic->auto_eoi_bitmap);
> else
> __clear_bit(vector, synic->auto_eoi_bitmap);
> +
> + auto_eoi_new = bitmap_weight(synic->auto_eoi_bitmap, 256);
> +
> + /* Hyper-V SynIC auto EOI SINTs are not compatible with APICV */
> + if (!auto_eoi_old && auto_eoi_new) {
> + if (atomic_inc_return(&hv->synic_auto_eoi_used) == 1)
> + kvm_request_apicv_update(vcpu->kvm, false,
> + APICV_INHIBIT_REASON_HYPERV);
> + } else if (!auto_eoi_new && auto_eoi_old) {
> + if (atomic_dec_return(&hv->synic_auto_eoi_used) == 0)
> + kvm_request_apicv_update(vcpu->kvm, true,
> + APICV_INHIBIT_REASON_HYPERV);
> + }
> }
>
> static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
> @@ -931,12 +950,6 @@ int kvm_hv_activate_synic(struct kvm_vcpu *vcpu, bool dont_zero_synic_pages)
>
> synic = to_hv_synic(vcpu);
>
> - /*
> - * Hyper-V SynIC auto EOI SINT's are
> - * not compatible with APICV, so request
> - * to deactivate APICV permanently.
> - */
> - kvm_request_apicv_update(vcpu->kvm, false, APICV_INHIBIT_REASON_HYPERV);
> synic->active = true;
> synic->dont_zero_synic_pages = dont_zero_synic_pages;
> synic->control = HV_SYNIC_CONTROL_ENABLE;
> @@ -2198,6 +2211,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
> ent->eax |= HV_X64_ENLIGHTENED_VMCS_RECOMMENDED;
> if (!cpu_smt_possible())
> ent->eax |= HV_X64_NO_NONARCH_CORESHARING;
> + if (enable_apicv)
> + ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
> /*
> * Default number of spinlock retry attempts, matches
> * HyperV 2016.
>

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