Re: [PATCH 62/67] KVM: SVM: Don't check vCPU's blocking status when toggling AVIC on/off

[Paolo Bonzini <pbonzini@redhat.com>]

On 4/4/25 21:39, Sean Christopherson wrote:
> Don't query a vCPU's blocking status when toggling AVIC on/off; barring
> KVM bugs, the vCPU can't be blocking when refrecing AVIC controls.  And if

refrecing -> refreshing

Paolo

> there are KVM bugs, ensuring the vCPU and its associated IRTEs are in the
> correct state is desirable, i.e. well worth any overhead in a buggy
> scenario.
> 
> Isolating the "real" load/put flows will allow moving the IOMMU IRTE
> (de)activation logic from avic_refresh_apicv_exec_ctrl() to
> avic_update_iommu_vcpu_affinity(), i.e. will allow updating the vCPU's
> physical ID entry and its IRTEs in a common path, under a single critical
> section of ir_list_lock.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/svm/avic.c | 65 +++++++++++++++++++++++------------------
>   1 file changed, 37 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 0425cc374a79..d5fa915d0827 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -838,7 +838,7 @@ static void avic_update_iommu_vcpu_affinity(struct kvm_vcpu *vcpu, int cpu)
>   		WARN_ON_ONCE(amd_iommu_update_ga(cpu, ir->data));
>   }
>   
> -void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> +static void __avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   {
>   	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
>   	int h_physical_id = kvm_cpu_get_apicid(cpu);
> @@ -854,16 +854,6 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   	if (WARN_ON_ONCE(vcpu->vcpu_id * sizeof(entry) >= PAGE_SIZE))
>   		return;
>   
> -	/*
> -	 * No need to update anything if the vCPU is blocking, i.e. if the vCPU
> -	 * is being scheduled in after being preempted.  The CPU entries in the
> -	 * Physical APIC table and IRTE are consumed iff IsRun{ning} is '1'.
> -	 * If the vCPU was migrated, its new CPU value will be stuffed when the
> -	 * vCPU unblocks.
> -	 */
> -	if (kvm_vcpu_is_blocking(vcpu))
> -		return;
> -
>   	/*
>   	 * Grab the per-vCPU interrupt remapping lock even if the VM doesn't
>   	 * _currently_ have assigned devices, as that can change.  Holding
> @@ -898,31 +888,33 @@ void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   	spin_unlock_irqrestore(&svm->ir_list_lock, flags);
>   }
>   
> -void avic_vcpu_put(struct kvm_vcpu *vcpu)
> +void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> +{
> +	/*
> +	 * No need to update anything if the vCPU is blocking, i.e. if the vCPU
> +	 * is being scheduled in after being preempted.  The CPU entries in the
> +	 * Physical APIC table and IRTE are consumed iff IsRun{ning} is '1'.
> +	 * If the vCPU was migrated, its new CPU value will be stuffed when the
> +	 * vCPU unblocks.
> +	 */
> +	if (kvm_vcpu_is_blocking(vcpu))
> +		return;
> +
> +	__avic_vcpu_load(vcpu, cpu);
> +}
> +
> +static void __avic_vcpu_put(struct kvm_vcpu *vcpu)
>   {
>   	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
>   	struct vcpu_svm *svm = to_svm(vcpu);
>   	unsigned long flags;
> -	u64 entry;
> +	u64 entry = svm->avic_physical_id_entry;
>   
>   	lockdep_assert_preemption_disabled();
>   
>   	if (WARN_ON_ONCE(vcpu->vcpu_id * sizeof(entry) >= PAGE_SIZE))
>   		return;
>   
> -	/*
> -	 * Note, reading the Physical ID entry outside of ir_list_lock is safe
> -	 * as only the pCPU that has loaded (or is loading) the vCPU is allowed
> -	 * to modify the entry, and preemption is disabled.  I.e. the vCPU
> -	 * can't be scheduled out and thus avic_vcpu_{put,load}() can't run
> -	 * recursively.
> -	 */
> -	entry = svm->avic_physical_id_entry;
> -
> -	/* Nothing to do if IsRunning == '0' due to vCPU blocking. */
> -	if (!(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK))
> -		return;
> -
>   	/*
>   	 * Take and hold the per-vCPU interrupt remapping lock while updating
>   	 * the Physical ID entry even though the lock doesn't protect against
> @@ -942,7 +934,24 @@ void avic_vcpu_put(struct kvm_vcpu *vcpu)
>   		WRITE_ONCE(kvm_svm->avic_physical_id_table[vcpu->vcpu_id], entry);
>   
>   	spin_unlock_irqrestore(&svm->ir_list_lock, flags);
> +}
>   
> +void avic_vcpu_put(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * Note, reading the Physical ID entry outside of ir_list_lock is safe
> +	 * as only the pCPU that has loaded (or is loading) the vCPU is allowed
> +	 * to modify the entry, and preemption is disabled.  I.e. the vCPU
> +	 * can't be scheduled out and thus avic_vcpu_{put,load}() can't run
> +	 * recursively.
> +	 */
> +	u64 entry = to_svm(vcpu)->avic_physical_id_entry;
> +
> +	/* Nothing to do if IsRunning == '0' due to vCPU blocking. */
> +	if (!(entry & AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK))
> +		return;
> +
> +	__avic_vcpu_put(vcpu);
>   }
>   
>   void avic_refresh_virtual_apic_mode(struct kvm_vcpu *vcpu)
> @@ -983,9 +992,9 @@ void avic_refresh_apicv_exec_ctrl(struct kvm_vcpu *vcpu)
>   	avic_refresh_virtual_apic_mode(vcpu);
>   
>   	if (activated)
> -		avic_vcpu_load(vcpu, vcpu->cpu);
> +		__avic_vcpu_load(vcpu, vcpu->cpu);
>   	else
> -		avic_vcpu_put(vcpu);
> +		__avic_vcpu_put(vcpu);
>   
>   	/*
>   	 * Here, we go through the per-vcpu ir_list to update all existing