[PATCH 2/2] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Vitaly Kuznetsov]

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@xxxxxxxxxx>
---
 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 ffafdb7b24cb..98391c9c18df 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..e1ecc2143794 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 (kvm_x86_ops.apicv_supported())
+				ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
 			/*
 			 * Default number of spinlock retry attempts, matches
 			 * HyperV 2016.
-- 
2.31.1