diff mbox series

[v3,09/12] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

Message ID 20210802183329.2309921-10-mlevitsk@redhat.com (mailing list archive)
State New, archived
Headers show
Series My AVIC patch queue | expand

Commit Message

Maxim Levitsky Aug. 2, 2021, 6:33 p.m. UTC 
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.

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

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           | 27 +++++++++++++++++++++------
 2 files changed, 24 insertions(+), 6 deletions(-)

Comments

Paolo Bonzini Aug. 3, 2021, 8:47 a.m. UTC | #1 
On 02/08/21 20:33, Maxim Levitsky wrote:
> Maxim:
>     - always set HV_DEPRECATING_AEOI_RECOMMENDED in kvm_get_hv_cpuid,
>       since this feature can be used regardless of AVIC

I tend to agree here.  While AutoEOI does have a minor performance 
improvement, I suspect that APICv will be available in most cases where 
Windows guest performance matters to that point.

Paolo
Paolo Bonzini Aug. 3, 2021, 9:01 a.m. UTC | #2 
On 02/08/21 20:33, Maxim Levitsky wrote:
> 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.
> 
> Maxim:
>     - always set HV_DEPRECATING_AEOI_RECOMMENDED in kvm_get_hv_cpuid,
>       since this feature can be used regardless of AVIC
> 
> 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           | 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 80323b5fb20a..55b1f79d9c43 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -988,6 +988,9 @@ 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..638f3c559623 100644
> --- 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 +100,25 @@ 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,
> @@ -933,12 +952,6 @@ 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 +2479,8 @@ 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.
> 

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Paolo Bonzini Aug. 3, 2021, 9:11 a.m. UTC | #3 
On 02/08/21 20:33, Maxim Levitsky wrote:
> +	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);

Hmm no, Reviewed-by rescinded.  This is racy, you cannot use atomics. 
The check for zero needs to happen within the lock.

The easiest way is to have a __kvm_request_apicv_update function that 
leaves it to the caller to take the lock.  Then synic_update_vector can do

	if (!!auto_eoi_old == !!auto_eoi_new)
		return;

	mutex_lock(&kvm->apicv_update_lock);
	bool was_active = hv->synic_auto_eoi_used;
	if (auto_eoi_new)
		hv->synic_auto_eoi_used++;
	else
		hv->synic_auto_eoi_used--;

	if (!!hv->synic_auto_eoi_used != !!was_active)
		__kvm_request_apicv_update(vcpu->kvm,
					   !!hv->synic_auto_eoi_used,
					   APICV_INHIBIT_REASON_HYPERV);
	mutex_unlock(&kvm->apicv_update_lock);

Please add a note to synic_auto_eoi_used that it is protected by 
apicv_update_lock.

Thanks,

Paolo
diff mbox series

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 80323b5fb20a..55b1f79d9c43 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -988,6 +988,9 @@  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..638f3c559623 100644
--- 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 +100,25 @@  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,
@@ -933,12 +952,6 @@  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 +2479,8 @@  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.