Re: [RFC PATCH v4 2/6] vfio: Introduce vGPU display irq type

From: Alex Williamson <alex.williamson@redhat.com>
To: "kraxel@redhat.com" <kraxel@redhat.com>
Cc: "Zhang, Tina" <tina.zhang@intel.com>,
	"Lu, Kechen" <kechen.lu@intel.com>,
	"intel-gvt-dev@lists.freedesktop.org" 
	<intel-gvt-dev@lists.freedesktop.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"zhenyuw@linux.intel.com" <zhenyuw@linux.intel.com>,
	"Lv, Zhiyuan" <zhiyuan.lv@intel.com>,
	"Wang, Zhi A" <zhi.a.wang@intel.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"Yuan, Hang" <hang.yuan@intel.com>
Subject: Re: [RFC PATCH v4 2/6] vfio: Introduce vGPU display irq type
Date: Fri, 2 Aug 2019 08:38:59 -0600	[thread overview]
Message-ID: <20190802083859.0fb0f05e@x1.home> (raw)
In-Reply-To: <20190802133531.4zwsjltvjisq4sfz@sirius.home.kraxel.org>

On Fri, 2 Aug 2019 15:35:31 +0200
"kraxel@redhat.com" <kraxel@redhat.com> wrote:

>   Hi,
> 
> > > > Couldn't you expose this as another capability within the IRQ_INFO return
> > > > data?  If you were to define it as a macro, I assume that means it would be
> > > > hard coded, in which case this probably becomes an Intel specific IRQ, rather
> > > > than what appears to be framed as a generic graphics IRQ extension.  A new
> > > > capability could instead allow the vendor to specify their own value, where
> > > > we could define how userspace should interpret and make use of this value.
> > > > Thanks,    
> > > Good suggestion. Currently, vfio_irq_info is used to save one irq
> > > info. What we need here is to use it to save several events info.
> > > Maybe we could figure out a general layout of this capability so that
> > > it can be leveraged by others, not only for display irq/events.  
> > 
> > You could also expose a device specific IRQ with count > 1 (ie. similar
> > to MSI/X) and avoid munging the eventfd value, which is not something
> > we do elsewhere, at least in vfio.  Thanks,  
> 
> Well, the basic idea is to use the eventfd value to signal the kind of
> changes which did happen, simliar to IRQ status register bits.
> 
> So, when the guest changes the primary plane, the mdev driver notes
> this.  Same with the cursor plane.  On vblank (when the guests update
> is actually applied) the mdev driver wakes the eventfd and uses eventfd
> value to signal whenever primary plane or cursor plane or both did
> change.
> 
> Then userspace knows which planes need an update without an extra
> VFIO_DEVICE_QUERY_GFX_PLANE roundtrip to the kernel.
> 
> Alternatively we could have one eventfd for each change type.  But given
> that these changes are typically applied at the same time (vblank) we
> would have multiple eventfds being signaled at the same time.  Which
> doesn't look ideal to me ...

Good point, looking at the bits in the eventfd value seems better than
a flood of concurrent interrupts.  Thanks,

Alex