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

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Date:	Tue, 8 Mar 2016 22:54:25 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	joro@...tes.org, bp@...en8.de, gleb@...nel.org,
	alex.williamson@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, wei@...hat.com,
	sherry.hurwitz@....com
Subject: Re: [PART1 RFC v2 06/10] svm: Add interrupt injection via AVIC

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.
 A comment explaining this optimization would be nice.  I'm thinking
 about a race where we don't send the doorbell even though the VCPU is
 in guest mode, because vcpu->mode was read before writing APIC_IRR.)

>> +
>> +	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.

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.