Re: [PATCH v2 8/8] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2021-07-13 at 17:20 +0300, 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: in case APICv/AVIC usage is impossible, AutoEOI
> is still preferred.

> 
> Maxim:
>    - added SRCU lock drop around call to kvm_request_apicv_update
>    - 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           | 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 e11d64aa0bcd..f900dca58af8 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);
> +}

Well turns out that this patch still doesn't work (on this
weekend I found out that all my AVIC enabled VMs hang on reboot).

I finally found out what prompted me back then to make srcu lock drop
in synic_update_vector conditional on whether the write was done
by the host.
 
Turns out that while KVM_SET_MSRS does take the kvm->srcu lock,
it stores the returned srcu index in a local variable and not
in vcpu->srcu_idx, thus the lock drop in synic_toggle_avic
doesn't work.
 
So it is likely that I have seen it not work, and blamed 
KVM_SET_MSRS for not taking the srcu lock which was a wrong assumption.
 
I am more inclined to fix this by just tracking if we hold the srcu
lock on each VCPU manually, just as we track the srcu index anyway,
and then kvm_request_apicv_update can use this to drop the srcu
lock when needed.


> +
>  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;

Vitally, I would like to hear your opinion on this change I also made to your code.
I think that we should always expose HV_DEPRECATING_AEOI_RECOMMENDED as a supported
HV cpuid bit regardless of AVIC, so that qemu could set it regardless of AVIC
in the kernel, even if this is not optimal.


Best regards,
	Maxim Levitsky

>  			/*
>  			 * Default number of spinlock retry attempts, matches
>  			 * HyperV 2016.