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Re: [Qemu-devel] [PATCH 19/22] qapi: add QMP put-event command

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 19/22] qapi: add QMP put-event command
Date: Wed, 09 Mar 2011 08:26:15 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8

On 03/09/2011 07:58 AM, Avi Kivity wrote:
On 03/09/2011 03:48 PM, Anthony Liguori wrote:
+[ 'put-event', {'tag': 'int'}, {}, 'none' ]

Why is tag an int?

It's a handle so the type doesn't matter as long as I can make sure values are unique. ints are easier to work with because they don't require memory allocation.

I think it's nicer for the client to use a string. Instead of a global ID allocator, it can use unique IDs or unique prefixes + local IDs. Should also aid a little in debugging.

handle's are opaque to clients.

I don't have a huge objection to using strings and it may make sense to do a prefix like you're suggesting. I won't do that initially but since handles are opaque, we can make that change later.

  don't we use strings for command ids and similar?

id's can be any valid JSON value.

But a handle is not the same thing as an id.

Why not?

I hope handles are client-provided?

No, they are generated by the server. It makes sense because really a handle is a marshalled version of a signal. The signal accessor looks something like this:

{ 'BLOCK_IO_ERROR': { 'device': 'str', 'action': 'str', 'operation': 'str' } }
[ 'get-block-io-error-event': {}, 'BLOCK_IO_ERROR' }

The way we marshal a 'BLOCK_IO_ERROR' type is by generating a unique handle and returning that.

While this looks like an int on the wire, at both the server and libqmp level, it looks like a BlockIoErrorEvent object. So in QEMU:

BlockIoErrorEvent *qmp_get_block_io_error_event(Error **errp)

And in libqmp:

BlockIoErrorEvent *libqmp_get_block_io_error_event(QmpSession *sess, Error **errp)

Also could be better named, disconnect-event or unlisten-event.

I was going for symmetry with the signal accessors which are typically in the format 'get-block-io-error-event'.

Maybe it would be better to do 'connect-block-io-error-event' and 'disconnect-event'?


But I'm confused, do we have a per-event command on the wire? Or just C stubs?

Ignoring default events, you'll never see an event until you execute a signal accessor function. When you execute this function, you will start receiving the events and those events will carry a tag containing the handle returned by the signal accessor.

Within libqmp, any time you execute a signal accessor, a new signal object is created of the appropriate type. When that object is destroyed, you send a put-event to stop receiving the signal.

When you connect to a signal object (via libqmp), you don't execute the signal accessor because the object is already receiving the signal.

Default events (which exist to preserve compatibility) are a set of events that are automatically connected to after qmp_capabilities is executed. Because these connections are implicit, they arrive without a handle in the event object.

At this point, libqmp just ignores default events. In the future, I'd like to add a command that can be executed before qmp_capabilities that will avoid connecting to default events.


Anthony Liguori

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