Re: [PATCH 4/4] qapi: introduce CONFIG_READ event

[Markus Armbruster]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> On 17.10.23 18:00, Markus Armbruster wrote:
>> Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:
>> 
>>> Send a new event when guest reads virtio-pci config after
>>> virtio_notify_config() call.
>>>
>>> That's useful to check that guest fetched modified config, for example
>>> after resizing disk backend.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

[...]

>>> diff --git a/qapi/qdev.json b/qapi/qdev.json
>>> index 2468f8bddf..37a8785b81 100644
>>> --- a/qapi/qdev.json
>>> +++ b/qapi/qdev.json
>>> @@ -329,3 +329,25 @@
>>>   # Since: 8.2
>>>   ##
>>>   { 'command': 'x-device-sync-config', 'data': {'id': 'str'} }
>>> +
>>> +##
>>> +# @X_CONFIG_READ:
>>> +#
>>> +# Emitted whenever guest reads virtio device config after config change.
>>> +#
>>> +# @device: device name
>>> +#
>>> +# @path: device path
>>> +#
>>> +# Since: 5.0.1-24
>>> +#
>>> +# Example:
>>> +#
>>> +# <- { "event": "X_CONFIG_READ",
>>> +#      "data": { "device": "virtio-net-pci-0",
>>> +#                "path": "/machine/peripheral/virtio-net-pci-0" },
>>> +#      "timestamp": { "seconds": 1265044230, "microseconds": 450486 } }
>>> +#
>>> +##
>>> +{ 'event': 'X_CONFIG_READ',
>>> +  'data': { '*device': 'str', 'path': 'str' } }
>>
>> The commit message talks about event CONFIG_READ, but you actually name
>> it x-device-sync-config.
>
> will fix
>
>> I figure you use x- to signify "unstable".  Please use feature flag
>> 'unstable' for that.  See docs/devel/qapi-code-gen.rst section
>> "Features", in particular "Special features", and also the note on x- in
>> section "Naming rules and reserved names".
>
> OK, will do.
>
> Hmm, it say
>
>    Names beginning with ``x-`` used to signify "experimental".  This
>    convention has been replaced by special feature "unstable".
>
> "replaced".. So, I should use "unstable" flag without "x-" prefix? Can't find 
> an example. Seems "unstable" always used together with "x-".

True.

The "x-" prefix originated with qdev properties.  First use might be
commit f0c07c7c7b4.  The convention wasn't documented then, but QOM/qdev
properties have always been a documentation wasteland.  It then spread
to other places, and eventually to the QAPI schema.  Where we try pretty
hard to document things properly.  We documented the "x-" prefix in
commit e790e666518:

    Any name (command, event, type, field, or enum value) beginning with
    "x-" is marked experimental, and may be withdrawn or changed
    incompatibly in a future release.

Minor pain point: when things grow up from experimental to stable, we
have to rename.

The convention didn't stop us from naming non-experimental things x-FOO,
e.g. QOM property "x-origin" in commit 6105683da35.  Made it to the QAPI
schema in commit 8825587b53c.  Point is: the prefix isn't a reliable
marker for "unstable".

Since I needed a reliable marker for my "set policy for unstable
interfaces" feature (see CLI option -compat), I created special feature
flag "unstable", and dropped the "x-" convention for the QAPI schema.

Renaming existing "x-" names felt like pointless churn, so I didn't.

I'm not objecting to new names starting with "x-".  Nor am I objecting
to feature 'unstable' on names that don't start with "x-".

I guess "x-" remains just fine for things we don't intend to make stable
at some point.  The "x-" can remind humans "this is unstable" better
than a feature flag can (for machines, it's the other way round).

For things we do intend (hope?) to make stable, I wouldn't bother with
the "x-".

Clearer now?

> Also, nothing said about events. Is using "X_" wrong idea? Should it be 
> x-SOME_EVENT instead?

Since this is the first unstable event, there is no precedent.  Let's
use no prefix, and move on.

>> The name CONFIG_READ feels overly generic for something that makes sense
>> only with virtio devices.
>
> Hmm, right.. I think, we can say same thing about DEVICE_UNPLUG_GUEST_ERROR.

That one came to be as a generalization of existing MEM_UNPLUG_ERROR and
a concrete need to signal CPU unplug errors.  Demonstrates "unplug guest
errors" can happen for different kinds of devices.  So we went with a
generic event we can use for all of them.

This doesn't seem to be the case for this patch's event.  Thoughts?

> So, what about DEVICE_GUEST_READ_CONFIG ?
>
>> 
>>> diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
>>> index b485375049..d0f022e925 100644
>>> --- a/softmmu/qdev-monitor.c
>>> +++ b/softmmu/qdev-monitor.c
>>> @@ -1252,3 +1252,8 @@ void qdev_hotplug_device_on_event(DeviceState *dev)
>>>       dev->device_on_event_sent = true;
>>>       qapi_event_send_x_device_on(dev->id, dev->canonical_path);
>>>   }
>>> +
>>> +void qdev_config_read_event(DeviceState *dev)
>>> +{
>>> +    qapi_event_send_x_config_read(dev->id, dev->canonical_path);
>>> +}
>>