Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, May 24, 2021 at 05:55:18PM +0200, Paolo Bonzini wrote:
> On 10/05/21 19:26, Marcelo Tosatti wrote:
> > +void vmx_pi_start_assignment(struct kvm *kvm)
> > +{
> > +	struct kvm_vcpu *vcpu;
> > +	int i;
> > +
> > +	if (!irq_remapping_cap(IRQ_POSTING_CAP))
> > +		return;
> > +
> > +	/*
> > +	 * Wakeup will cause the vCPU to bail out of kvm_vcpu_block() and
> > +	 * go back through vcpu_block().
> > +	 */
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		if (!kvm_vcpu_apicv_active(vcpu))
> > +			continue;
> > +
> > +		kvm_vcpu_wake_up(vcpu);
> 
> Would you still need the check_block callback, if you also added a
> kvm_make_request(KVM_REQ_EVENT)?
> 
> In fact, since this is entirely not a hot path, can you just do
> kvm_make_all_cpus_request(kvm, KVM_REQ_EVENT) instead of this loop?
> 
> Thanks,
> 
> Paolo

Hi Paolo,

Don't think so:

int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
{
        return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu);
}

static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
{
        int ret = -EINTR;
        int idx = srcu_read_lock(&vcpu->kvm->srcu);

        if (kvm_arch_vcpu_runnable(vcpu)) {
                kvm_make_request(KVM_REQ_UNHALT, vcpu);  <---- don't want KVM_REQ_UNHALT
                goto out;
        }
        if (kvm_cpu_has_pending_timer(vcpu))
                goto out;
        if (signal_pending(current))
                goto out;

        ret = 0;
out:
        srcu_read_unlock(&vcpu->kvm->srcu, idx);
        return ret;
}

See previous discussion:


Date: Wed, 12 May 2021 14:41:56 +0000                                                                                   
From: Sean Christopherson <seanjc@google.com>                                                                           
To: Marcelo Tosatti <mtosatti@redhat.com>                                                                               
Cc: Peter Xu <peterx@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org, Alex Williamson             
        <alex.williamson@redhat.com>, Pei Zhang <pezhang@redhat.com>                                                    
Subject: Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device                        

On Tue, May 11, 2021, Marcelo Tosatti wrote:
> > The KVM_REQ_UNBLOCK patch will resume execution even any such event                                                 
>                                                                                                                       
>                                                 even without any such event                                           
>                                                                                                                       
> > occuring. So the behaviour would be different from baremetal.                                                       

I agree with Marcelo, we don't want to spuriously unhalt the vCPU.  It's legal,
albeit risky, to do something like

       	hlt
       	/* #UD to triple fault if this CPU is awakened. */
       	ud2

when offlining a CPU, in which case the spurious wake event will crash the guest.