[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Replace GSource with AioContext for chardev
From: |
Daniel P . Berrangé |
Subject: |
Re: Replace GSource with AioContext for chardev |
Date: |
Thu, 9 Apr 2020 14:24:41 +0100 |
User-agent: |
Mutt/1.13.3 (2020-01-12) |
On Thu, Apr 09, 2020 at 03:16:01PM +0200, Marc-André Lureau wrote:
> Hi
>
> On Thu, Apr 9, 2020 at 2:46 PM Coiby Xu <address@hidden> wrote:
> >
> >
> > Hi,
> >
> > I'm now implementing vhost-user block device backend
> > https://patchew.org/QEMU/address@hidden/
> > and want to use chardev to help manage vhost-user client connections
> > and read socket message. However there are two issues that need to be
> > addressed.
> >
> > Firstly, chardev isn't suitable for the case when exported drive is
> > run in an IOThread because for mow chardev use GSource to dispatch
> > socket fd events. So I have to specify which IOThread the exported
> > drive is using when launching vhost-user block device backend,
> > for example, the following syntax will be used,
> >
> > -drive file=file.img,id=disk -device
> > virtio-blk,drive=disk,iothread=iothread0 \
> > -object
> > vhost-user-blk-server,node-name=disk,chardev=mon1,iothread=iothread0 \
> > -object iothread,id=iothread0 \
> > -chardev
> > socket,id=mon1,path=/tmp/vhost-user-blk_vhost.socket,server,nowait
> >
> > then iothread_get_g_main_context(IOThread *iothread) has to be called
> > to run the gcontext in IOThread. If we use AioContext to dispatch socket
> > fd events, we needn't to specify IOThread twice. Besides aio_poll is faster
> > than g_main_loop_run.
> >
> > Secondly, socket chardev's async read handler (set through
> > qemu_chr_fe_set_handlers) doesn't take the case of socket short read
> > into consideration. I plan to add one which will make use
> > qio_channel_yield.
> >
> > According to
> > [1] Improving the QEMU Event Loop - Linux Foundation Events
> > http://events17.linuxfoundation.org/sites/events/files/slides/Improving%20the%20QEMU%20Event%20Loop%20-%203.pdf
> >
> > "Convert chardev GSource to aio or an equivalent source" (p.30) should have
> > been finished. I'm curious why the plan didn't continue. If it's desirable,
> > I'm going to finish the leftover work to resolve the aforementioned two
> > issues.
>
> Converting all chardevs to Aio might be challenging, and doesn't bring
> much benefits imho.
>
> Perhaps a better approach would be to rely on a new chardev API to
> steal the chardev underlying fd or QIO... (mostly keeping -chardev for
> CLI/QMP compatibility reason - although breaking some chardev features
> that imho aren't compatible with all use cases, like replay, muxing,
> swapping etc). The chardev should probably be removed after that...
Yeah, I feel like it was a mistake for us to wire up many of our features
to chardevs. We mostly did it because -chardev provides a pre-existing
syntax for TCP/UNIX sockets and we didn't want to invent new CLI args.
IMHO this was a mistake in retrospect.
Unfortunately the -chardev API is absolutely terrible for any usage that
actually cares about the connection based semantics. Witness the horrible
hacks we do for re-connect and re-try when failing to initially connect
in vhost-user net code.
For features in QEMU where the only desirable chardev backend is one with
connection based, socket semantics, I think we would be better off using
the QIOChannel APIs directly and completely avoiding the chardev code.
Regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
- Replace GSource with AioContext for chardev, Coiby Xu, 2020/04/09
- Re: Replace GSource with AioContext for chardev, Marc-André Lureau, 2020/04/09
- Re: Replace GSource with AioContext for chardev,
Daniel P . Berrangé <=
- Re: Replace GSource with AioContext for chardev, Markus Armbruster, 2020/04/14
- Re: Replace GSource with AioContext for chardev, Daniel P . Berrangé, 2020/04/14
- Re: Replace GSource with AioContext for chardev, Paolo Bonzini, 2020/04/14
- Re: Replace GSource with AioContext for chardev, Kevin Wolf, 2020/04/14
- Re: Replace GSource with AioContext for chardev, Daniel P . Berrangé, 2020/04/15