Re: [virtio-dev] [PATCH 0/1] Add "Group Identifier" to virtio PCI capabilities.

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

On Mon, Jun 04, 2018 at 11:44:40AM -0500, Venu Busireddy wrote:
> On 2018-06-02 00:10:31 +0300, Michael S. Tsirkin wrote:
> > On Sat, Jun 02, 2018 at 12:09:35AM +0300, Michael S. Tsirkin wrote:
> > > On Fri, Jun 01, 2018 at 03:50:43PM -0500, Venu Busireddy wrote:
> > > > On 2018-06-01 23:03:16 +0300, Michael S. Tsirkin wrote:
> > > > > On Fri, Jun 01, 2018 at 12:01:26PM -0500, Venu Busireddy wrote:
> > > > > > On 2018-06-01 18:42:06 +0300, Michael S. Tsirkin wrote:
> > > > > > > On Wed, May 23, 2018 at 11:16:12AM -0400, Venu Busireddy wrote:
> > > > > > > > During live migration involving passthrough devices, the guest needs
> > > > > > > > to know which virtio device will be a fail-over device for a given
> > > > > > > > passthrough device.
> > > > > > > > 
> > > > > > > > Extending the virtio specification with a new "Group Identifier"
> > > > > > > > capability allows qemu to set up the grouping at the time the guest
> > > > > > > > is created. The "Group Identifier" can be as simple as a number, or an
> > > > > > > > UUID. The driver can use the group identifier to pair the virtio device
> > > > > > > > with the passthrough device. The passthrough device can contain the
> > > > > > > > group identifier in the PCIe bridge to which it is attached.
> > > > > > > > 
> > > > > > > > Venu Busireddy (1):
> > > > > > > >   Add "Group Identifier" to virtio PCI capabilities.
> > > > > > > > 
> > > > > > > >  content.tex | 43 +++++++++++++++++++++++++++++++++++++++++++
> > > > > > > >  1 file changed, 43 insertions(+)
> > > > > > > 
> > > > > > > Is this a PCI thing, or can this somehow be used with non-PCI
> > > > > > 
> > > > > > This is applicable to all virtio PCI devices, but not to non-PCI
> > > > > > devices.
> > > > > > 
> > > > > > > devices? If PCI, we can just add a PCI UUID capability to
> > > > > > > virtio without need to worry about the spec.
> > > > > > 
> > > > > > What is a "PCI UUID capability?" "PCI Local Bus Specification
> > > > > > Revision 3.0, Appendix H" does not list any such capability.
> > > > > > 
> > > > > > Regards,
> > > > > > 
> > > > > > Venu
> > > > > 
> > > > > Sorry that I'm unclear. What I meant is the
> > > > > PCI Express serial number capability (0003h).
> > > > 
> > > > 0003h is the VPD capability.
> > > 
> > > You are talking about the PCI capabilities.
> > > I'm talking about PCI Extended capabilities.
> > > 
> > > These are not the same thing.
> > > 
> > > > Serial Number ("SN") is part of the VPD. QEMU
> > > > does not support VPD, and adding the VPD capability to QEMU is lot more
> > > > involved than storing the UUID in Vendor-Specific capability.
> > > 
> > > Formatting VPD is not so hard really. Another question would be how hard
> > > is it to parse VPD in guests? It might be hard e.g. for windows drivers.
> > > 
> > > > Do you strongly believe that adding VPD is required? Or, can we live
> > > > with using Vendor-Specific capability? The bridge device is going to be
> > > > QEMU specific device anyway, so there should be no ambiguity of mixing
> > > > up with other bridges.
> > > > 
> > > > Thanks,
> > > > 
> > > > Venu
> > > 
> > > I'm inclined to say use the serial number extended capability.
> > > If not, I'd look at how hard it is to parse VPD.
> > > 
> > > Standard cap is better in that guests will be able to show it more
> > > easily.
> > 
> > But yes, it's not a very strong preference. If you strongly
> > believe we must use a vendor specific cap, I can live with
> > that decision.
> 
> Let me summarize the discussion so far...
> 
> One of the suggestions was to use the "Device Serial Number" PCIe
> extended capability. The problem I see with that is that, by default,
> virtio devices are exposed as PCI devices, not as PCIe devices (bit 4 in
> the Status Register in the configuration space is set to 0). As a result,
> there is no extended configuration space.
> 
> Do we want to modify QEMU to expose all virtio devices as PCI express
> devices? This may have backward compatibility issues!

Not all but it's a reasonable limitation to say that if you want to use
failover you use a pci express device.

> The other suggestion was to use VPD. Implementing VPD support into
> QEMU will be a lot harder than adding a new virtio PCI capability. A
> quick look at the VPD format, and the handshake mechanism involved in
> exchanging the VPD, will be sufficient to see how complicated it will be.

Well QEMU does not need to compile a generic VPD - just generate a
specific record.  It's not all that complicated.

> 
> Therefore, shall we go ahead with the current proposal to add
> VIRTIO_PCI_CAP_GROUP_ID_CFG to the virtio PCI capability list?

I won't nack it, I just wanted you to consider the alternatives.

> Please note that VIRTIO_PCI_CAP_GROUP_ID_CFG is a capability that "may be
> present." If we enhance QEMU to support VPD in the future, we can switch
> to using that. The driver in the guest should be coded to look for the
> VPD, and if it not present, look for the VIRTIO_PCI_CAP_GROUP_ID_CFG
> capability.
> 
> Thanks,
> 
> Venu

I'm not sure we should code up things that we didn't even test.
How likely are they to work flawlessly?

-- 
MST

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