KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

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.

Maxim:
   - always set HV_DEPRECATING_AEOI_RECOMMENDED in kvm_get_hv_cpuid,
     since this feature can be used regardless of AVIC

Paolo:
   - use arch.apicv_update_lock to protect the hv->synic_auto_eoi_used
     instead of atomic ops

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210810205251.424103-12-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 20a3ffe..28c85c8 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -981,6 +981,12 @@ struct kvm_hv {
        /* How many vCPUs have VP index != vCPU index */
        atomic_t num_mismatched_vp_indexes;
 
+       /*
+        * How many SynICs use 'AutoEOI' feature
+        * (protected by arch.apicv_update_lock)
+        */
+       unsigned int 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 0b38f94..fe4a027 100644 (file)
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -88,6 +88,10 @@ 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;
 
@@ -96,10 +100,30 @@ 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);
+
+       if (!!auto_eoi_old == !!auto_eoi_new)
+               return;
+
+       mutex_lock(&vcpu->kvm->arch.apicv_update_lock);
+
+       if (auto_eoi_new)
+               hv->synic_auto_eoi_used++;
+       else
+               hv->synic_auto_eoi_used--;
+
+       __kvm_request_apicv_update(vcpu->kvm,
+                                  !hv->synic_auto_eoi_used,
+                                  APICV_INHIBIT_REASON_HYPERV);
+
+       mutex_unlock(&vcpu->kvm->arch.apicv_update_lock);
 }
 
 static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
@@ -933,12 +957,6 @@ 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;
@@ -2476,6 +2494,8 @@ 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;
+
+                       ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
                        /*
                         * Default number of spinlock retry attempts, matches
                         * HyperV 2016.