Re: [PATCH v9 12/18] vfio: Register/unregister irq_bypass_producer

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 27 Apr 2016 01:32:32 +0000
"Wu, Feng" <feng.wu@intel.com> wrote:

> > -----Original Message-----
> > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > Sent: Wednesday, April 27, 2016 4:08 AM
> > To: Wu, Feng <feng.wu@intel.com>
> > Cc: pbonzini@redhat.com; joro@8bytes.org; mtosatti@redhat.com;
> > eric.auger@linaro.org; kvm@vger.kernel.org; iommu@lists.linux-
> > foundation.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v9 12/18] vfio: Register/unregister irq_bypass_producer
> > 
> > On Fri, 18 Sep 2015 22:29:50 +0800
> > Feng Wu <feng.wu@intel.com> wrote:
> > 
> >  @@ -360,6 +361,14 @@ static int vfio_msi_set_vector_signal(struct
> >  vfio_pci_device *vdev,  
> > >  		return ret;
> > >  	}
> > >
> > > +	vdev->ctx[vector].producer.token = trigger;
> > > +	vdev->ctx[vector].producer.irq = irq;
> > > +	ret = irq_bypass_register_producer(&vdev->ctx[vector].producer);
> > > +	if (unlikely(ret))
> > > +		dev_info(&pdev->dev,
> > > +		"irq bypass producer (token %p) registeration fails: %d\n",
> > > +		vdev->ctx[vector].producer.token, ret);
> > > +
> > >  	vdev->ctx[vector].trigger = trigger;
> > >
> > >  	return 0;  
> > 
> > Digging back into the IRQ producer/consumer thing, I'm not sure how we
> > should be handling a failure here, but it turns out that what we have
> > is pretty sub-optimal.  Any sort of testing on AMD hits this dev_info
> > because kvm_arch_irq_bypass_add_producer() returns -EINVAL without
> > kvm_x86_ops->update_pi_irte which is only implemented for vmx.  Clearly
> > we don't want to spew confusing error messages for a feature that does
> > not exist.
> > 
> > The easiest option is to simply make this error silent, but should
> > registering a producer/consumer really fail due to a mismatch on the
> > other end or should the __connect sequence fail silently, which both
> > ends would know about (if they care) due to the add/del handshake
> > between them?  Perhaps for now we simply need a stable suitable fix to
> > silence the dev_info above, but longer term, registration shouldn't
> > fail for mismatches like this.  Thoughts?  Thanks,  
> 
> Can we just return 0 when kvm_x86_ops->update_pi_irte is NULL in
> kvm_arch_irq_bypass_add_producer?

Yeah, that may be the best way to go, only return error for actual
failures, not for simple lack of a bypass mechanism.  This is
consistent with what update_pi_irte does when running on hardware
or configurations without PI.  Thanks,

Alex