Re: Maximum QMP reply size

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> 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).

We could put size limits on xp; most Humans will only dump a few kB
maximum like that, any larger and you can dump to file.

Dave

> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK