linux-kernel - Re: [PATCH v9 12/18] vfio: Register/unregister irq_bypass

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Message-ID: <20160428104028.63ff2109@t450s.home>
Date:	Thu, 28 Apr 2016 10:40:28 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	"Wu, Feng" <feng.wu@...el.com>
Cc:	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"joro@...tes.org" <joro@...tes.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"eric.auger@...aro.org" <eric.auger@...aro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/18] vfio: Register/unregister irq_bypass_producer

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

> > -----Original Message-----
> > From: Alex Williamson [mailto:alex.williamson@...hat.com]
> > Sent: Wednesday, April 27, 2016 4:08 AM
> > To: Wu, Feng <feng.wu@...el.com>
> > Cc: pbonzini@...hat.com; joro@...tes.org; mtosatti@...hat.com;
> > eric.auger@...aro.org; kvm@...r.kernel.org; iommu@...ts.linux-
> > foundation.org; linux-kernel@...r.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@...el.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