Re: [libvirt] [PATCH V1 1/4] Add support for QEMU -add-fd support detection

On 01/29/2013 05:16 PM, Stefan Berger wrote: We have two options: 1. Try to support both qemu 1.2 and qemu 1.3. For qemu 1.3, we can use the command line (which feels simpler to me), but for qemu 1.2 we would have to do everything through QMP. This means that we'd have to write code that does not attach _any_ -drive arguments, and instead start 'qemu -s', hotplug all the drives, and then start the domain. But if we can get things working for 1.2 via hotplug only, then there is no need to use the command line for 1.3 - use a single interface for it all. This method is easy to probe - does the 'add-fd' QMP command exist in 'query-commands'? 2. Not support fd passing unless targeting qemu 1.3 or newer. When targeting 1.2 and older, use the command line as always, and state that qemu will use open() and the user has to use virt_use_nfs. With 1.3, use -add-fd on the command line, so that we don't have to worry about hot-plugging disks after 'qemu -s' on initial start; we'd still have to wire up QMP command support for hotplug, but at least we don't have to refactor the command line computation into delaying things to hotplug. Since there is no 'query-config-schema' command in 1.3, we'll have to hack in some other capability check; for this I have two ideas: a) straight version check (if 'query-version' reports 1.3 or newer, assume -add-fd exists); this does not backport well. b) during the initial QMP probes, if 'add-fd' exists, try calling 'add-fd' with a specified fdset-id argument and /dev/null as an fd; qemu 1.2 will reject the call (in that version, qemu insisted on managing set ids itself; you could only pass fdset-id when adding to an existing set); qemu 1.3 and later will accept it (the management app libvirt can choose its own allocation pattern for set ids). I'm leaning towards 2b, so I'll code that up as part of my QMP work. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org