Re: [Qemu-devel] Use getaddrinfo for migration

[Amos Kong <akong@redhat.com>]

On 02/03/12 18:41, Daniel P. Berrange wrote:
> On Fri, Mar 02, 2012 at 02:25:36PM +0400, Michael Tokarev wrote:
>> Not a reply to the patch but a general observation.
>>
>> I noticed that the tcp migration uses gethostname
>> (or getaddrinfo after this patch) from the main
>> thread - is it really the way to go?  Note that
>> DNS query which is done may block for a large amount
>> of time.  Is it really safe in this context?  Should
>> it resolve the name in a separate thread, allowing
>> guest to run while it is doing that?
>>
>> This question is important for me because right now
>> I'm evaluating a network-connected block device driver
>> which should do failover, so it will have to resolve
>> alternative name(s) at runtime (especially since list
>> of available targets is dynamic).
>>
>>  From one point, _usually_, the delay there is very
>> small since it is unlikely you'll do migration or
>> failover overseas, so most likely you'll have the
>> answer from DNS handy.  But from another point, if
>> the DNS is malfunctioning right at that time (eg,
>> one of the two DNS resolvers is being rebooted),
>> the delay even from local DNS may be noticeable.
>
> Yes, I think you are correct - QEMU should take care to ensure that
> DNS resolution can not block the QEMU event loop thread.
>
> There is the GLib extension (getaddrinfo_a) which does async DNS
> resolution, but for sake of portability it is probably better
> to use a thread to do it.


I've prepared a V2 according to Kevin's comment,
https://github.com/kongove/qemu/commits/master

But I don't know how to process the getaddrinfo issue,
which steps should be done by a thread?
anyone can give a hint? thanks.

== migrate steps ==
0. main_loop, qemu_iohandler_poll
1. get migration command from qemu monitor
2. parse host/port, get an address list by getaddrinfo()
3. connect server
4. check status and return to main_loop (step 0)

(VMstate data is transmitted in background)

main_loop_wait()
...
  \- do_migrate()
    \- tcp_start_outgoing_migration()
      \- tcp_client_start()
        \- parse_host_port_info()
          \- getaddrinfo()


-- 
			Amos.