Re: [PATCH v2 5/5] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/05/21 16:43, Vitaly Kuznetsov wrote:
> 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@redhat.com>

Should it also disable APICv unconditionally if 
HV_DEPRECATING_AEOI_RECOMMENDED is not in the guest CPUID?  That should 
avoid ping-pong between enabled and disabled APICv even in pathological 
cases that we cannot think about.

Paolo

> ---
>   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 bf5807d35339..5e03ab4c0e4f 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..89e7d5b99279 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 (enable_apicv)
> +				ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED;
>   			/*
>   			 * Default number of spinlock retry attempts, matches
>   			 * HyperV 2016.
>