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Re: [Qemu-devel] Dropping the MONITOR_CMD_ASYNC

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Dropping the MONITOR_CMD_ASYNC
Date: Mon, 12 Dec 2011 15:23:35 +0000

On Mon, Dec 12, 2011 at 12:39 PM, Gerd Hoffmann <address@hidden> wrote:
> On 12/12/11 13:10, Stefan Hajnoczi wrote:
>> On Mon, Dec 12, 2011 at 11:29 AM, Gerd Hoffmann <address@hidden> wrote:
>>> On 12/12/11 11:18, Stefan Hajnoczi wrote:
>>>> On Sun, Dec 11, 2011 at 10:29 AM, Alon Levy <address@hidden> wrote:
>>>>> On Thu, Dec 08, 2011 at 05:45:44PM -0200, Luiz Capitulino wrote:
>>>>>> Hi there,
>>>>>> I'm about to completely drop the MONITOR_CMD_ASYNC API, but it turns out 
>>>>>> that
>>>>>> the command client_migrate_info uses it. That's a legacy interface and 
>>>>>> has to
>>>>>> be dropped, no command should be using it...
>>>>>> Something tells me that if I just drop it (and change the command to use 
>>>>>> the
>>>>>> regular interface), bad things will happen. Am I right? :)
>>>>> The monitor command client_migrate_info needs to complete after getting
>>>>> an ACK message from the currently connected spice client (this is the
>>>>> only case where this is required - if there is no client then the
>>>>> MONITOR_CMD_ASYNC API won't be used). This in turn requires the main
>>>>> thread to perform select and call the callback that will process this
>>>>> ACK. That's why the MONITOR_CMD_ASYNC API was used.
>>>>> I'm not aware of any other way to do this, I'll be glad for any help
>>>>> here. Also, I understand this is not what is not true async, since one
>>>>> would expect a true async interface to support multiple in flight
>>>>> monitor commands. If there is any ETA or existing way to do this we
>>>>> could change the implementation of client_migrate_info.
>>>> Is it possible to use a QMP event to signal completion instead of a
>>>> MONITOR_CMD_ASYNC command?
>>> Problem is this breaks the qemu <-> libvirt interface.
>> I had the same issue with the block_job_cancel command, which Adam
>> Litke and Eric Blake helped us fix and find a backward-compatible
>> libvirt solution for:
>> http://www.redhat.com/archives/libvir-list/2011-November/msg01351.html
> Isn't going to fly as waiting for completion isn't optional in that
> case.  Workflow is this:
> (1) libvirt issues client_migrate_info command.
> (2) qemu forwards it to spice-server, which in turn forwards it to
>    the spice client (if connected).
> (3) spice client connects to the migration target machine.
> (4) spice client signals completion to spice-server, which in turn
>    notifies qemu.
> (5) qemu calls the monitor completion callback, libvirt gets
>    client_migrate_info result.
> (6) libvirt issues migrate command.
> The problem is that (3) must be finished before (6) because qemu on the
> target side doesn't accept incoming tcp connections any more once the
> migration started.
> I don't see a way to switch this to qmp events without breaking old
> libvirt versions managing new qemu.

I don't see a solution in this case either.  Looks like libvirt
supports this command since 0.9.2 so it's not a good idea to just yank

It might be possible for the QEMU client_migrate_info handler to run a
nested event loop in the legacy libvirt case.  This would suck since
the VM is unresponsive while waiting for spice migration to complete.
New libvirt would call the async version of the command which is
well-behaved and uses a QMP event to signal completion.


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