Re: [Qemu-devel] client_migrate_info - do we need a new command?

[Avi Kivity]

On 12/13/2011 08:19 PM, Anthony Liguori wrote:
> In our call today, Avi asked that we evaluate whether the interface
> for client_migrate_info is the Right Interface before we introduce a
> new command to work around the fact that async commands are broken.
>
> I looked into this today and here's what I came to.
>

Thanks.

> 1) What are the failure scenarios?
>
> The issue is qerror_report().  Roughly speaking, qerror_report either
> prints to stderr or it associates an error with the current monitor
> command.
>
> The problem with this is that qerror_report() is used all over the
> code base today and if an error occurs in a device that has nothing to
> do with the command, instead of printing to stderr, the command will
> fail with a bizarre error reason (even though it really succeeded).
>
> 2) Does the command have the right semantics?
>
> The command has the following doc:
>
> client_migrate_info
> ------------------

Somewhat poorly named - commands should be verbs.

>
> Set the spice/vnc connection info for the migration target.  The
> spice/vnc
> server will ask the spice/vnc client to automatically reconnect using the
> new parameters (if specified) once the vm migration finished
> successfully.
>
> Arguments:
>
> - "protocol":     protocol: "spice" or "vnc" (json-string)
> - "hostname":     migration target hostname (json-string)
> - "port":         spice/vnc tcp port for plaintext channels (json-int,
> optional)
> - "tls-port":     spice tcp port for tls-secured channels (json-int,
> optional)
> - "cert-subject": server certificate subject (json-string, optional)
>
> Example:
>
> -> { "execute": "client_migrate_info",
>      "arguments": { "protocol": "spice",
>                     "hostname": "virt42.lab.kraxel.org",
>                     "port": 1234 } }
> <- { "return": {} }
>
> Originally, the command was a normal sync command and my understanding
> is that it simply posted notification to the clients.  Apparently,
> users of the interface need to actually know when the client has Ack'd
> this operation because otherwise it's racy since a disconnect may
> occur before the client processes the redirection.
>
> OTOH, that means that what we really need is 1) tell connected clients
> that they need to redirect 2) notification when/if connected clients
> are prepared to redirect.
>
> The trouble with using a async command for this is that the time
> between (1) & (2) may be arbitrarily long.  Since most QMP clients
> today always use a NULL tag, that effectively means the monitor is
> blocked for an arbitrarily long time while this operation is in flight.
>
> I don't know if libspice uses a timeout for this operation, but if it
> doesn't, this could block arbitrarily long.  Even with tagging, we
> don't have a way to cancel in flight commands so blocking for
> arbitrary time periods is problematic.
>
> I think splitting this into two commands, one that requests the
> clients to redirect and then an event that lets a tool know that the
> clients are ready to migrate ends up being nicer.  It means that we
> never end up with a blocked QMP session and clients are more likely to
> properly deal with the fact that an event may take arbitrarily long to
> happen.
>
> Clients can also implement their own cancel logic by choosing to stop
> waiting for an event to happen and then ignoring spurious events.
>
> So regardless of the async issue, I think splitting this command is
> the right thing to do long term.
>

Nothing is solved by the split; it has exactly the same issues.  If an
error occurs during execution of the command (say, a timeout), you need
to capture the error and return it during the event.  If the command
consumes resources or takes a lock, you need to send a cancellation
request or it will continue executing.  You've simply renamed the return
part of the RPC to an event.

-- 
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