| Date | Mon, 25 Oct 2021 16:34:43 +0200 | Subject | Re: [PATCH v2 39/43] KVM: VMX: Don't do full kick when triggering posted interrupt "fails" | From | Paolo Bonzini <> |
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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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