[libvirt] [PATCHv2 01/12] pci: eliminate unused driver arg from virPCIDeviceDetach

[Laine Stump]

On 06/25/2013 05:50 AM, Daniel P. Berrange wrote:
> On Mon, Jun 24, 2013 at 11:05:27PM -0400, Laine Stump wrote:
>> The driver arg to virPCIDeviceDetach is no longer used (the name of
>> the stub driver is now set in the virPCIDevice object, and
>> virPCIDeviceDetach retrieves it from there). Remove it.
> What happens when libvirtd is restarted ? Are we correctly preserving
> the original value across restart, or are we capable of auto-detecting
> the correct value again ?

That's a good question, for two reasons: 1) I didn't know the answer for
sure when you asked it, and 2) I went back over the code, learned the
answer, and it was a good answer :-)

Originally the stubDriverName in the virPCIDevice object was only used
for a short period - during qemu's "device Preparation" it will create a
virPCIDeviceList from the virDomainHostdevDef list, then later iterate
through the virPCIDeviceList to detach all the devices from their host
drivers; during this detach loop, the original viDomainHostdevDef isn't
available, so an indicator of which stub to bind to must be stashed in
the virPCIDevice object when it is created.

Later it became useful to be able to rely on the stubDriverName always
being correct (at least in the activePciHostdevs list; it doesn't matter
for inactivePciHostdevs). Fortunately that list is recreated whenever
libvirt restarts by calling qemuUpdateActivePciHostdevs() for every
active domain. That function also updates stubDriverName for each active
hostdev based on the config (which must be the actual driver in use,
otherwise we would have encountered an error when the device was
originally being attached).

So, the answer is yes, we auto-set the correct value in the virPCIDevice
object when libvirtd restarts.


However, this entire topic brings up a question that I've had for awhile
- what happens to the "inactivePciHostdevs" list when libvirt restarts?
This is the list of devices that have been detached from the host by
libvirt's virNodeDevice API, but are not currently in use by any domain.
This list isn't stored in any status file, and is not reconstructed when
libvirtd restarts (the information to do that doesn't exist). I *think*
the answer is that it doesn't matter, because libvirt will just use the
device anyway, then put it on the inactive list when it's done with it.
But that suggests the question "Then why have the inactive list?" I
don't know the answer to that one, but it does seem like either we
should be reconstructing inactivePciHostdevs when libvirtd restarts, or
we should just give up and not use (or maintain) it.