[libvirt] [RFC PATCH v2 2/3] qemu: RDMA migration support using 'rdma' URI

[Michael R. Hines]

On 04/04/2014 11:47 PM, Eric Blake wrote:
> On 04/04/2014 12:19 AM, Michael R. Hines wrote:
>
>>>>> @@ -2561,6 +2570,10 @@ virQEMUCapsInitQMPMonitor(virQEMUCapsPtr
>>>>> qemuCaps,
>>>>>        if (qemuCaps->version >= 1006000)
>>>>>            virQEMUCapsSet(qemuCaps, QEMU_CAPS_DEVICE_VIDEO_PRIMARY);
>>>>>    +    if (qemuCaps->version >= 2000000)
>>>>> +        virQEMUCapsSet(qemuCaps, QEMU_CAPS_MIGRATE_QEMU_RDMA);
>>>>> +
>>>>> +
>>>> And here we need a better check for rdma migration. What if someone
>>>> compiles QEMU without RDMA support?
>>> Better than hard-coding it to a version string is to probe the results
>>> of query-migrate-capabilities and only setting the capability if the
>>> resulting list includes rdma-pin-all, as that will serve as a reliable
>>> witness of qemu being new enough to support rdma without an x- prefix.
>>>
>> These comments I don't understand: Why can't we depend on the
>> version number here? Isn't that what it was designed for?
> No.  Features get backported downstream all the time, to something that
> ostensibly fails the version number check.  For example, RHEL 6 qemu
> claims to be 0.10, but has backported many features that are much closer
> to upstream qemu 1.7.  Libvirt basing a feature check on a version
> number will guess wrong if RHEL 7 backports the upstream qemu 2.0
> feature to whatever 1.x version of downstream qemu lives in RHEL.
> Whereas probing for the _feature_ (by calling query-migrate-capabilities
> and looking for rdma-pin-all) will work for ALL qemu builds, regardless
> of whether that qemu calls itself 2.0 or not.

I had no idea there was so much backporting =).

Acknowledged. Will have to get a patch upstream to fix this - there is
currently no (current) method in the migration capabilities to conditionally
enable them depending on the existence of a particular feature.

I'll have to cook something up with a #define which only makes the
capability visible if the feature was compiled in.....