[libvirt] Add support for QEMU file descriptor sets

[Eric Blake]

On 01/29/2013 03:23 AM, Daniel P. Berrange wrote:
> On Mon, Jan 28, 2013 at 02:54:28PM -0700, Eric Blake wrote:
>> On 01/26/2013 09:17 PM, Stefan Berger wrote:
>>>> In a subsequent patch we now test inside that function whether the new
>>>> command line parameter is available using the capability (from another
>>>> patch) and create fd=/dev/set/10 and vhostfd=/dev/set/20 respectively
>>>> *and* already add "-add-fd set=10,fd=21" and "-add-fd set=20,fd=23" to
>>>> virCommand using the functions I already posted (as the lowest layer).
>>>>
>>>> Would it be that simple, or do we need to add more parameters to
>>>> -add-fd set=10,fd=21,xyz under certain circumstances?
>>
>> Ideally, we'd also want to use the opaque parameter of each -add-fd
>> call, so that when we later use QMP commands to query what fdsets exist,
>> the opaque parameter can tell us what filenames we passed in (it makes
>> bookkeeping easier if you know that fd 5 was tied to /dev/tpm, compared
>> to just knowing that it is an open fd but not what it came from).
> 
> That implies that you trust the output from QEMU monitor commands
> to be telling you the truth. We can't do that, so IMHO we can't
> rely on the 'opaque' parameter.

Fair enough - libvirt will have to track in vm->priv anything that it
needs for reconstructing state, even across libvirtd restarts.  Still,
we _ought_ to populate the opaque member with useful information, so
that debugging is easier (that is, while libvirt can't rely on
query-fdsets, a developer using 'virsh qemu-monitor-command' on a
trusted guest certainly should be able to).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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