Messages in this thread |  | | Date | Thu, 28 Oct 2021 00:09:25 +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 27/10/21 18:04, 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. >> >> What this smp_wmb() pair with, is the smp_mb__after_atomic in >> kvm_check_request(KVM_REQ_EVENT, vcpu). > > I don't think that's correct. There is no kvm_check_request() in the relevant path. > kvm_vcpu_exit_request() uses kvm_request_pending(), which is just a READ_ONCE() > without a barrier.
Ok, we are talking about two different set of barriers. This is mine:
- smp_wmb() in kvm_make_request() pairs with the smp_mb__after_atomic() in kvm_check_request(); it ensures that everything before the request (in this case, pi_pending = true) is seen by inject_pending_event.
- pi_test_and_set_on() orders the write to ON after the write to PIR, pairing with vmx_sync_pir_to_irr and ensuring that the bit in the PIR is seen.
And this is yours:
- pi_test_and_set_on() _also_ orders the write to ON before the read of vcpu->mode, pairing with vcpu_enter_guest()
- kvm_make_request() however does _not_ order the write to vcpu->requests before the read of vcpu->mode, even though it's needed. Usually that's handled by kvm_vcpu_exiting_guest_mode(), but in this case vcpu->mode is read in kvm_vcpu_trigger_posted_interrupt.
So vmx_deliver_nested_posted_interrupt() is missing a smp_mb__after_atomic(). It's documentation only for x86, but still easily done in v3.
Paolo
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