[PATCH 10/10] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Maxim Levitsky <mlevitsk@redhat.com>]

From: Vitaly Kuznetsov <vkuznets@redhat.com>

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.

Maxim:
   - Drop SRCU lock around the call to kvm_request_apicv_update
     to avoid deadlock.
   - Always set HV_DEPRECATING_AEOI_RECOMMENDED in kvm_get_hv_cpuid,
     since this feature can be used regardless of APICv.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 arch/x86/include/asm/kvm_host.h |  3 +++
 arch/x86/kvm/hyperv.c           | 34 +++++++++++++++++++++++++++------
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 9ed5c55be352..9eb78ec6a290 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -956,6 +956,9 @@ 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 b07592ca92f0..6bf47a583d0e 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -85,9 +85,22 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
 	return false;
 }
 
+
+static void synic_toggle_avic(struct kvm_vcpu *vcpu, bool activate)
+{
+	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
+	kvm_request_apicv_update(vcpu->kvm, activate,
+			APICV_INHIBIT_REASON_HYPERV);
+	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
+}
+
 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 +109,23 @@ 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)
+			synic_toggle_avic(vcpu, false);
+	} else if (!auto_eoi_new && auto_eoi_old) {
+		if (atomic_dec_return(&hv->synic_auto_eoi_used) == 0)
+			synic_toggle_avic(vcpu, true);
+	}
 }
 
 static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
@@ -933,12 +959,6 @@ 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;
@@ -2466,6 +2486,8 @@ 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.
-- 
2.26.3