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Re: [Qemu-devel] Re: [RFC][PATCH v1 05/12] qapi: fix handling for null-r

From: Michael Roth
Subject: Re: [Qemu-devel] Re: [RFC][PATCH v1 05/12] qapi: fix handling for null-return async callbacks
Date: Mon, 28 Mar 2011 15:42:24 -0500
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv: Gecko/20110303 Thunderbird/3.1.9

On 03/28/2011 01:27 PM, Anthony Liguori wrote:
On 03/28/2011 12:59 PM, Michael Roth wrote:

For a command like this, I can't imagine ever wanting to extend the
return value...

I think this is another topic, but also one we should hash out a bit

Currently the plan is that the C API not expose asynchronicity,
underneath the covers the library will issue the request, then do a
blocking read for the response. So the API call will block till
completion, and no other command's will be serviced through the same
underlying session until it is completed or cancelled.

No, that's just the patches as they stand today.

The medium term goal is to have twin APIs--one API that is synchronous
and another that is asynchronous. The asynchronous version will mirror
how asynchronous commands are dispatched within QEMU. That is, for
query-block, you'll have:

typedef void (*QueryBlockFunc)(void *opaque, BlockInfo *retval, Error

void libqmp_async_query_block(QmpSession *sess, Error **errp,
QueryBlockFunc *cb, void *opaque);

The challenge with async library commands is that you need to think
carefully about how you interact with the socket. You can make a glib
mainloop be a prerequisite, or you can have a set of function pointers
that let you implement your own main loop.

But since there isn't a pressing need for this in the short term, it's
easier to just delay this.

For the JSON-based clients, the behavior is different. You have an
optional tag you can pass in for an async command, and after issuing
one, you can immediately begin issuing other async or non-async
commands. As a result, the responses you receive will not necessarily
be in FIFO order.

There is no such thing as a "JSON-based client". QMP is not self
describing enough to implement this with a pure transport client.
Another language implementation (which I think you mean) would still
need to solve the same problem as libqmp.

By JSON-based I mean interacting directly with the QMP server socket via, say, `socat unix-connect:/tmp/qmp.sock readline`. I think this is what you're describing as being the wire protocol.

What I mean is that the wire protocol currently supports more than the libqmp C API does in terms of being able to handle multiple in-flight requests.

My thought was simply that if the C API provided by libqmp (and other language bindings) would always be synchronous, then the wire protocol, and the server-side handling of it, could potentially be simplified by enforcing FIFO ordering and not needing to handle multiple in-flight requests.

But if the long-term goal is to provide for asynchronous APIs then that probably doesn't make any sense.

The upside to this is you can implement async commands on the client
side without using separate threads, and can exploit some level of
concurrency being able to do work within a session while a slower
host->guest command completes. The downsides are that:

1) There is some inconsistency between this and the C API semantics.

You still need to pass a continuation to implement an event-based
interface regardless of the language you're using. The function
pointer/opaque parameter is a continuation in C.

2) The "optional" tags are basically required tags, at least for async
commands, unless the client side does something to force synchronicity.

One option would be to disable the QMP session's read handler once a
JSON object is received, and not re-enable it until we send the
response. This would enforce FIFO-ordering. It might also add reduce
the potential for a client being able to blow up our TX buffers by
issuing lots of requests and not handling the responses in a timely
enough manner (have seen this just from piping responses to stdout).

No, we're mixing up wire semantics with client/server semantics.

These are all completely different things.

The wire semantics are:

1) All commands are tagged. Untagged commands have an implicit tag
(let's refer to it as the psuedo-tag).

2) Until a command is completed, a tag cannot be reused. This is also
true for the psuedo-tag.

3) There is no guarantee about command completion order.

4) If a client happens to use the same tag for all commands, the client
ends up enforcing a completion order because the server is only ever
processing one command at a time.

Is this supposed to be the current behavior? In early testing I noticed that not including a tag, and issuing an async command that never completed, still allowed for me to get responses for subsequent, tagless/pseudo-tagged requests.

The server semantics are:

1) All tags are preserved including the psuedo-tag. This is required by
the protocol.

2) Most commands are implemented by immediately dispatching a function
and then computing the return value and immediately putting on the
socket buffer.

3) Some commands are implemented by delaying the computation of the
return value. When this happens (which is an indeterminate amount of
time later), the data will be put on the socket buffer.

4) Which commands are handled by (2) vs. (3) are transparent to the client.

The (current) client semantics are:

1) All commands are tagged with the psuedo-tag which enforces that only
one command can be in flight at a time. In the future, this interface
will support threading and use different tags such that two threads can
be used to send simultaneous commands.

2) A second interface will be implemented that provides an event-based
interface whereas each command is passed a continuation. Commands will
use different tags to support this interface.

3) The reason to have both interfaces is to support the two most common
models of concurrency, event-based concurrency and thread based

Notice that I said nothing about 'C' in the above. It's equally true to
a C or Python client.

This clears things up a lot for me, thanks.


Anthony Liguori


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