Re: [PATCH v2 37/43] KVM: SVM: Unconditionally mark AVIC as running on vCPU load (with APICv)

[Maxim Levitsky]

On Fri, 2021-10-08 at 19:12 -0700, Sean Christopherson wrote:
> Always mark the AVIC as "running" on vCPU load when the AVIC is enabled and
> drop the vcpu_blocking/unblocking hooks that toggle "running".  There is no
> harm in keeping the flag set for a wee bit longer when a vCPU is blocking,
> i.e. between the start of blocking and being scheduled out.  At worst, an
> agent in the host will unnecessarily signal the doorbell, but that's
> already the status quo in KVM as the "running" flag is set the entire time
> a vCPU is loaded, not just when it's actively running the guest.
> 
> In addition to simplifying the code, keeping the "running" flag set longer
> can reduce the number of VM-Exits due to incomplete IPI delivery.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 53 +++++++++++++----------------------------
>  arch/x86/kvm/svm/svm.c  |  8 -------
>  arch/x86/kvm/svm/svm.h  |  3 ---
>  3 files changed, 17 insertions(+), 47 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index b43b05610ade..213f5223f63e 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -967,6 +967,15 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	int h_physical_id = kvm_cpu_get_apicid(cpu);
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> +	/* TODO: Document why the unblocking path checks for updates. */
> +	if (kvm_vcpu_is_blocking(vcpu) &&
> +	    kvm_check_request(KVM_REQ_APICV_UPDATE, vcpu)) {
> +		kvm_vcpu_update_apicv(vcpu);
> +
> +		if (!kvm_vcpu_apicv_active(vcpu))
> +			return;
> +	}
> +
>  	/*
>  	 * Since the host physical APIC id is 8 bits,
>  	 * we can support host APIC ID upto 255.
> @@ -974,19 +983,21 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	if (WARN_ON(h_physical_id > AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK))
>  		return;
>  
> +	/*
> +	 * Unconditionally mark the AVIC as "running", even if the vCPU is in
> +	 * kvm_vcpu_block().  kvm_vcpu_check_block() will detect pending IRQs
> +	 * and bail out of the block loop, and if not, avic_vcpu_put() will
> +	 * set the AVIC back to "not running" when the vCPU is scheduled out.
> +	 */
>  	entry = READ_ONCE(*(svm->avic_physical_id_cache));
>  	WARN_ON(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK);
>  
>  	entry &= ~AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK;
>  	entry |= (h_physical_id & AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK);
> -
> -	entry &= ~AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
> -	if (svm->avic_is_running)
> -		entry |= AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
> +	entry |= AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
>  
>  	WRITE_ONCE(*(svm->avic_physical_id_cache), entry);
> -	avic_update_iommu_vcpu_affinity(vcpu, h_physical_id,
> -					svm->avic_is_running);
> +	avic_update_iommu_vcpu_affinity(vcpu, h_physical_id, true);
>  }
>  
>  void avic_vcpu_put(struct kvm_vcpu *vcpu)
> @@ -1001,33 +1012,3 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
>  	entry &= ~AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK;
>  	WRITE_ONCE(*(svm->avic_physical_id_cache), entry);
>  }
> -
> -/*
> - * This function is called during VCPU halt/unhalt.
> - */
> -static void avic_set_running(struct kvm_vcpu *vcpu, bool is_run)
> -{
> -	struct vcpu_svm *svm = to_svm(vcpu);
> -
> -	svm->avic_is_running = is_run;
> -
> -	if (!kvm_vcpu_apicv_active(vcpu))
> -		return;
> -
> -	if (is_run)
> -		avic_vcpu_load(vcpu, vcpu->cpu);
> -	else
> -		avic_vcpu_put(vcpu);
> -}
> -
> -void svm_vcpu_blocking(struct kvm_vcpu *vcpu)
> -{
> -	avic_set_running(vcpu, false);
> -}
> -
> -void svm_vcpu_unblocking(struct kvm_vcpu *vcpu)
> -{
> -	if (kvm_check_request(KVM_REQ_APICV_UPDATE, vcpu))
> -		kvm_vcpu_update_apicv(vcpu);
> -	avic_set_running(vcpu, true);
> -}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 89077160d463..a1ca5707f2c8 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1433,12 +1433,6 @@ static int svm_create_vcpu(struct kvm_vcpu *vcpu)
>  	if (err)
>  		goto error_free_vmsa_page;
>  
> -	/* We initialize this flag to true to make sure that the is_running
> -	 * bit would be set the first time the vcpu is loaded.
> -	 */
> -	if (irqchip_in_kernel(vcpu->kvm) && kvm_apicv_activated(vcpu->kvm))
> -		svm->avic_is_running = true;
> -
>  	svm->msrpm = svm_vcpu_alloc_msrpm();
>  	if (!svm->msrpm) {
>  		err = -ENOMEM;
> @@ -4597,8 +4591,6 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.prepare_guest_switch = svm_prepare_guest_switch,
>  	.vcpu_load = svm_vcpu_load,
>  	.vcpu_put = svm_vcpu_put,
> -	.vcpu_blocking = svm_vcpu_blocking,
> -	.vcpu_unblocking = svm_vcpu_unblocking,
>  
>  	.update_exception_bitmap = svm_update_exception_bitmap,
>  	.get_msr_feature = svm_get_msr_feature,
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 7f5b01bbee29..652d71acfb6c 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -169,7 +169,6 @@ struct vcpu_svm {
>  	u32 dfr_reg;
>  	struct page *avic_backing_page;
>  	u64 *avic_physical_id_cache;
> -	bool avic_is_running;
>  
>  	/*
>  	 * Per-vcpu list of struct amd_svm_iommu_ir:
> @@ -529,8 +528,6 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec);
>  bool svm_dy_apicv_has_pending_interrupt(struct kvm_vcpu *vcpu);
>  int svm_update_pi_irte(struct kvm *kvm, unsigned int host_irq,
>  		       uint32_t guest_irq, bool set);
> -void svm_vcpu_blocking(struct kvm_vcpu *vcpu);
> -void svm_vcpu_unblocking(struct kvm_vcpu *vcpu);
>  
>  /* sev.c */
>  

Looks good. It is nice to get rid of all of this logic that was just making things more complicated.

Something else nice to do here which I didn't finish back then when I worked on avic, would be
to maybe rename avic_vcpu_load/avic_vcpu_put because those are also now run on avic inhibit/uninhibit.

Basically the 'svm_refresh_apicv_exec_ctrl' is the full avic activate/deactivate, while
avic_vcpu_load/avic_vcpu_put are the lighter weight partial avic activation/deactivation functions.

So minus the comment from Paolo about the updating avic on unblock which I missed back when I wrote
my patches:

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky