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SubjectRe: [PATCH v2 37/43] KVM: SVM: Unconditionally mark AVIC as running on vCPU load (with APICv)
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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@google.com>
> ---
> 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@redhat.com>

Best regards,
Maxim Levitsky


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