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 <coiby...@gmail.com> wrote: > > > > > > Hi, > > > > I'm now implementing vhost-user block device backend > > https://patchew.org/QEMU/20200309100342.14921-1-coiby...@gmail.com/ > > 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 :|