Re: [PATCH v3 21/26] KVM: SVM: Drop AVIC's intermediate avic_set_running() helper

Paolo Bonzini
2021-12-08T14:43:31Z

On 12/8/21 02:52, Sean Christopherson wrote:
> + /*
> + * Unload the AVIC when the vCPU is about to block,_before_ the vCPU
> + * actually blocks. The vCPU needs to be marked IsRunning=0 before the
> + * final pass over the vIRR via kvm_vcpu_check_block(). Any IRQs that
> + * arrive before IsRunning=0 will not signal the doorbell, i.e. it's
> + * KVM's responsibility to ensure there are no pending IRQs in the vIRR
> + * after IsRunning is cleared, prior to scheduling out the vCPU.

I prefer to phrase this around paired memory barriers and the usual
store/smp_mb/load lockless idiom:

/*
* Unload the AVIC when the vCPU is about to block, _before_
* the vCPU actually blocks.
*
* Any IRQs that arrive before IsRunning=0 will not cause an
* incomplete IPI vmexit on the source, therefore vIRR will also
* be checked by kvm_vcpu_check_block() before blocking. The
* memory barrier implicit in set_current_state orders writing
* IsRunning=0 before reading the vIRR. The processor needs a
* matching memory barrier on interrupt delivery between writing
* IRR and reading IsRunning; the lack of this barrier might be
* the cause of errata #1235).
*/

Is there any nuance that I am missing?

Paolo

> + */
> + avic_vcpu_put(vcpu);
> +