Re: [PATCH v2 39/43] KVM: VMX: Don't do full kick when triggering posted interrupt "fails"

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);