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>

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;
 };

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.