Re: Maximum QMP reply size

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Sep 06, 2022 at 03:38:54PM -0400, John Snow 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?

As a reference, libvirt arbitrarily chose 10 MB as the QMP reply
limit. It is huge enough it'll be hard to make a QMP reply exceed
that, but also still tiny in the context of managing VMs on a host
with GB's of RAM.  NB, this doesn't mean we allocate 10 MB every
time, it is just an upper bound - we only allocate what we actually
need.


With regards,
Daniel
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