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SubjectRe: [PATCH v2 39/43] KVM: VMX: Don't do full kick when triggering posted interrupt "fails"
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On 09/10/21 04:12, Sean Christopherson wrote:
> + /*
> + * The smp_wmb() in kvm_make_request() pairs with the smp_mb_*()
> + * after setting vcpu->mode in vcpu_enter_guest(), thus the vCPU
> + * is guaranteed to see the event request if triggering a posted
> + * interrupt "fails" because vcpu->mode != IN_GUEST_MODE.

This explanation doesn't make much sense to me. This is just the usual
request/kick pattern explained in
Documentation/virt/kvm/vcpu-requests.rst; except that we don't bother
with a "kick" out of guest mode because the entry always goes through
kvm_check_request (in the nVMX case) or sync_pir_to_irr (if non-nested)
and completes the delivery itself.

In other word, it is a similar idea as patch 43/43.

What this smp_wmb() pair with, is the smp_mb__after_atomic in
kvm_check_request(KVM_REQ_EVENT, vcpu). Setting the interrupt in the
PIR orders before kvm_make_request in this thread, and orders after
kvm_make_request in the vCPU thread.

Here, instead:

> + /*
> + * The implied barrier in pi_test_and_set_on() pairs with the smp_mb_*()
> + * after setting vcpu->mode in vcpu_enter_guest(), thus the vCPU is
> + * guaranteed to see PID.ON=1 and sync the PIR to IRR if triggering a
> + * posted interrupt "fails" because vcpu->mode != IN_GUEST_MODE.
> + */
> if (vcpu != kvm_get_running_vcpu() &&
> !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
> - kvm_vcpu_kick(vcpu);
> + kvm_vcpu_wake_up(vcpu);
>

it pairs with the smp_mb__after_atomic in vmx_sync_pir_to_irr(). As
explained again in vcpu-requests.rst, the ON bit has the same function
as vcpu->request in the previous case.

Paolo

> + */
> kvm_make_request(KVM_REQ_EVENT, vcpu);

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