Re: Examining device state via monitor for debugging

[Mark Cave-Ayland]

On 11/06/2024 06:49, Markus Armbruster wrote:

> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
>
> > Officialise the QMP command, use the existing
> > hmp_info_human_readable_text() helper.
>
> I'm not sure "officialise" is a word :)
>
> Taking a step back...  "info via" and its new QMP counterpart
> x-query-mos6522-devices dump device state.  I understand why examining
> device state via monitor can be useful for debugging.  However, we have
> more than 2000 devices in the tree.  Clearly, we don't want 2000 device
> state queries.  Not even 100.  Could we have more generic means instead?
>
> We could use QOM (read-only) properties to expose device state.
>
> If we use one QOM property per "thing", examining device state becomes
> quite tedious.  Also, you'd have to stop the guest to get a consistent
> view, and adding lots of QOM properties bloats the code.
>
> If we use a single, object-valued property for the entire state, we get
> to define the objects in QAPI.  Differently tedious, and bloats the
> generated code.
>
> We could use a single string-valued property.  Too much of an abuse of
> QOM?
>
> We could add an optional "dump state for debugging" method to QOM, and
> have a single query command that calls it if present.
>
> Thoughts?

I agree that there should be a better way of doing things here. The aim of the 
original "info via" series was to allow the command to be contained completely within 
mos6522.c, but unfortunately due to the way that qemu-options.hx works then you end 
up with #ifdef-fery or stubs to make all configuration combinations work.

As you point out ideally there should be a way for a QOM object to dynamically 
register its own monitor commands, which I think should help with this.

IIRC in the original thread Daniel or David proposed a new "debug" monitor command 
such that a device could register its own debug <foo> commands either via DeviceClass 
or a function called during realize that would return a HumanReadableText via QMP.

In terms of "info via" it is only used by developers for the 68k and PPC Mac machines 
so if it were to change from "info via" to "debug via" I don't see there would be a 
big problem with this.


ATB,

Mark.