Re: [PART1 RFC v2 06/10] svm: Add interrupt injection via AVIC

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/03/2016 22:54, Radim Krčmář wrote:
> 2016-03-07 16:36+0100, Paolo Bonzini:
>> On 04/03/2016 21:46, Suravee Suthikulpanit wrote:
>>> +static void svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
>>> +{
>>> +	struct vcpu_svm *svm = to_svm(vcpu);
>>> +
>>> +	kvm_lapic_set_vector(vec, avic_get_bk_page_entry(svm, APIC_IRR));
> 
> (I think that smp_mb here would make sense, even though we're fine now
>  thanks to re-checking vcpu->mode in kvm_vcpu_kick.

Right, though only a smp_mb__after_atomic() is required (which is a
compiler barrier).  It is similarly required in vmx.

Offtopic remark, kvm_make_request() and kvm_check_request() should
probably have a smp_wmb() and a smp_rmb().

>>> +
>>> +	if (vcpu->mode == IN_GUEST_MODE) {
>>> +		wrmsrl(SVM_AVIC_DOORBELL,
>>> +		       __default_cpu_present_to_apicid(vcpu->cpu));
>>> +	} else {
>>> +		kvm_vcpu_kick(vcpu);
>>> +	}
>>
>> You also need to add
>>
>> 	kvm_make_request(KVM_REQ_EVENT, vcpu);
>>
>> before the "if", similar to vmx_deliver_posted_interrupt.
> 
> KVM won't do anything in KVM_REQ_EVENT and I think that the request can
> be avoided because KVM already has to handle IRR writes from AVIC.

Doh, I've made the same remark already and you've given the same answer. :)

> And what about
>   [...]
>   else if (!vcpu->...->is_running)
>   	kvm_vcpu_kick(vcpu);
> 
> ?
> The kick isn't needed unless the VCPU is scheduled out.
> Or maybe just
>   if (vcpu->...->is_running)
>     wrmsrl()
>   else
>     kvm_vcpu_kick();
> ?
> Which doesn't use the information we have on top AVIC, making our logic
> a bit simpler.

Yes, both of this should work.  I like the latter.

In any case, the smp_mb__after_atomic() is necessary.

Paolo