Re: [Qemu-devel] [PATCH 0/2]: qemu-ga: make shutdown & suspend synchronous

[Eric Blake]

On 05/14/2012 11:06 AM, Michal Privoznik wrote:
> On 11.05.2012 21:19, Luiz Capitulino wrote:
>> The main motivation for this series is fixing two possible race conditions
>> in the guest-suspend-* API due to the complexity that arose from the way
>> we handle terminated children processes today. Full details in the first
>> patch.
>>
>> This series applies on top of my two other qemu-ga series submitted
>> previously:
>>
>>  http://lists.gnu.org/archive/html/qemu-devel/2012-05/msg00999.html
>>
>>  http://lists.gnu.org/archive/html/qemu-devel/2012-05/msg01507.html
>>
>>  qapi-schema-guest.json   |   56 ++++++++++------
>>  qapi/qmp-core.h          |   10 ++-
>>  qapi/qmp-dispatch.c      |    8 ++-
>>  qapi/qmp-registry.c      |    4 +-
>>  qemu-ga.c                |   40 ++++++------
>>  qga/commands-posix.c     |  162 
>> ++++++++++++++++++----------------------------
>>  qga/guest-agent-core.h   |    4 ++
>>  scripts/qapi-commands.py |   14 +++-
>>  8 files changed, 154 insertions(+), 144 deletions(-)
>>
> 
> Okay, this is definitely an enhancement and fix of bogus implementation.
> One thing that I'd like to ask is - how can user distinguish between
> these implementations. I am asking basically from libvirt POV.
> Because if I assume I am dealing with the previous implementation and
> thus waiting for the {'return':{}} before I can return form an API, but
> the GA actually uses the new implementation I will block endlessly.

I think the point was that you would block endlessly waiting for the
{'return':{}} anyways, even with the old implementation, because there
was no guarantee that the guest agent could issue the reply in a timely
manner.  Therefore, the only sane implementation in libvirt is to assume
that success will not be reported via the guest agent, and that libvirt
must _always_ probe for the listed side effects (a change in guest
status, qemu exiting, an event, or so forth), and optionally have a
timeout if the associated timeout does not occur in a reasonable time.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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