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

On 01/29/2013 05:16 PM, Stefan Berger wrote:
> On 01/29/2013 06:24 PM, Eric Blake wrote:
>> On 01/29/2013 09:52 AM, Stefan Berger wrote:
>>> Index: libvirt/src/qemu/qemu_capabilities.h
>>> ===================================================================
>>> --- libvirt.orig/src/qemu/qemu_capabilities.h
>>> +++ libvirt/src/qemu/qemu_capabilities.h
>>> @@ -165,6 +165,7 @@ enum qemuCapsFlags {
>>>       QEMU_CAPS_SCLP_S390          = 124, /* -device sclp* */
>>>       QEMU_CAPS_DEVICE_USB_SERIAL  = 125, /* -device usb-serial */
>>>       QEMU_CAPS_DEVICE_USB_NET     = 126, /* -device usb-net */
>>> +    QEMU_CAPS_ADD_FD             = 127, /* -add-fd */
>> Okay, so you are taking the approach that we insist on command line
>> support (qemu 1.3), not just the QMP command existing (qemu 1.2).  It's
>> easy to probe whether 'add-fd' QMP exists (query-command), but harder to
>> determine if command line support is present when using just QMP with
>> the current state of qemu.
> Would you then not use -add-fd on the command line but hotplug those
> devices instead?

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

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

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