Re: [Qemu-devel] chardev's and fd's in monitors

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

On Tue, Oct 18, 2016 at 12:32:02PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Wed, Oct 12, 2016 at 08:15:02PM +0100, Dr. David Alan Gilbert wrote:
> > > Hi,
> > >   I had a look at a couple of readline like libraries;
> > > editline and linenoise.  A difficulty with using them is that
> > > they both want fd's or FILE*'s; editline takes either but
> > > from a brief look I think it's expecting to extract the fd.
> > > That makes them tricky to integrate into qemu, where
> > > the chardev's hide a whole bunch of non-fd things; in particular
> > > tls, mux, ringbuffers etc.
> > > 
> > > If we could get away with just a FILE* then we could use fopencookie,
> > > but that's GNU only.
> > > 
> > > Is there any sane way of shepherding all chardev's into having an
> > > fd?
> > 
> > The entire chardev abstraction model exists precisely because we cannot
> > make all chardevs look like a single fd. Even those which are fd based
> > may have separate FDs for input and output.
> 
> Note that editline takes separate in/out streams, but it does want those streams
> to be FILE*'s.
> 
> > IMHO the only viable approach would be to enhance linenoise/editline to
> > not assume use of fd* or FILE * abstractions.
> 
> I think if it came to that then we'd probably end up sticking with what we
> had for a very long time; I'd assume it would take a long time before
> any mods we made to the libraries would come around to be generally useful.
> 
> > BTW, what is the actual thread issue you are facing ? Chardevs at least
> > ought to be usable from a separate thread, as long as each distinct
> > chardev object instance was only used from one thread at a time ?
> 
> Marc-André pointed that out; I hadn't realised they were thread safe.
> But what are the rules? You say 'only used from one thread at a time' -
> what happens if we have a mux and the different streams to the mux come
> from different threads?

Well there is no mutex locking on the CharDriverState objects, so the
exact rule is "you mustn't do anything from multiple threads that will
race on contents of CharDriverState". That's too fuzzy to be useful to
developers though, so I think the only sensible option right now is to
say any "top level" CharDriverState should only be touch from one thread
at a time. IOW, if you have a mux, that that rule would apply to the
mux itself and the various children it owns as if they were a single
unnit.

> My actual thoughts for threads came from a few sides:
>   a) Maybe I could have a shim thread that fed the editline fd from a chardev
>   b) I'd eventually like multiple monitor threads.

Can you expand on what you mean by multiple monitor threads ? Presumably
you're meaning a single monitor instance, with multiple threads processing
commands concurrently ?  If so, I think that ought to be fine even with
the current thread rules around chardevs. The processing of individual
monitor commands doesn't interact with the CharDriverState AFAIR, as we
have clean separation between parsing the incoming command, running the
command, and formatting the outgoing response. IOW, for a single monitor
it is still sufficient to have a single thread deal with all I/O for the
chardev - only the command execution needs to be delegated to other
threads, and those wouldn't be touching the chardev at all.

Regards,
Daniel
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