Re: Maximum QMP reply size

[Peter Maydell <peter.maydell@linaro.org>]

On Tue, 6 Sept 2022 at 20:41, John Snow <jsnow@redhat.com> wrote:
> Hi, I suspect I have asked this before, but I didn't write it down in
> a comment, so I forget my justification...
>
> In the QMP lib, we need to set a buffering limit for how big a QMP
> message can be -- In practice, I found that the largest possible
> response was the QAPI schema reply, and I set the code to this:
>
>     # Maximum allowable size of read buffer
>     _limit = (64 * 1024)
>
> However, I didn't document if this was a reasonable limit or just a
> "worksforme" one. I assume that there's no hard limit for the protocol
> or the implementation thereof in QEMU. Is there any kind of value here
> that would be more sensible than another?
>
> I'm worried that if replies get bigger in the future (possibly in some
> degenerate case I am presently unaware of) that the library default
> will become nonsensical.

There are some QMP commands which return lists of things
where we put no inherent limit on how many things there
are in the list, like qom-list-types. We'd have to be getting
a bit enthusiastic about defining types for that to get
up towards 64K's worth of response, but it's not inherently
impossible. I think using human-monitor-command to send
an 'xp' HMP command is also a way to get back an arbitrarily
large string (just ask for a lot of memory to be dumped).

-- PMM