Re: [PATCH RFC] virtio-pci: share config interrupt between virtio devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Sep 21, 2014 at 11:36:44AM +0200, Stefan Fritsch wrote:
> On Sunday 21 September 2014 11:09:14, Michael S. Tsirkin wrote:
> > On Thu, Sep 18, 2014 at 09:18:37PM +0200, Stefan Fritsch wrote:
> > > On Monday 01 September 2014 09:37:30, Michael S. Tsirkin wrote:
> > > > Why do we need INT#x?
> > > > How about setting IRQF_SHARED for the config interrupt
> > > > while using MSI-X? You'd have to read ISR to check that the
> > > > interrupt was intended for your device.
> > >
> > > 
> > >
> > > The virtio 0.9.5 spec says that ISR is "unused" when in MSI-X
> > > mode. I  don't think that you can depend on the device to set the
> > > configuration changed bit.
> > > The virtio 1.0 spec seems to have fixed that.
> > 
> > Yes, virtio 0.9.5 has this bug. But in practice qemu always set this
> > bit, so for qemu we could do that unconditionally.  Pekka's lkvm
> > tool doesn't unfortunately.  It's easy to fix that, but it would be
> > nicer to additionally probe for old versions of the tool, and
> > disable IRQF_SHARED in that case.
> 
> What about other implementations? I think Linux should try to conform 
> to the spec so that all device implementations which conform to the 
> spec just work.
> 
> One implementation that comes to mind is virtualbox. But from a quick 
> look at the source, it seems that it sets the ISR bit always, too. And 
> it uses qemu's subsystem vendor id.
> 
> But there are other implementations. For example bhyve.

I couldn't find any code in bhyve that sets VTCFG_ISR_CONF_CHANGED.
Maybe it doesn't generate config changed interrupts?

bhyve sets subsystem vendor to 0 apparently?
We could use that to detect it.

But maybe we should just make it a 1.0 only feature.

> 
> > AFAIK a subsystem vendor id does not cost money to register, but
> > only pci sig members can do this, and membership costs $3000.
> > Maybe we should combine all this with checking subsystem vendor id,
> > and only implement the optimization if it matches qemu, for now.
> > This needs some thought.
> 
> Maybe the virtio spec should include a way to query the vendor that 
> does not involve the pci sig. Maybe use a string? Then no registry 
> would be necessary.

We can make the requirement for the vendor specific ID stronger in 1.0,
SHOULD instead of MAY.

But it seems that people will still copy-paste working code
across hypervisors, I'm not sure this can be helped.

-- 
MST