Daniel P. Berrangé <berra...@redhat.com> writes: > 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.
How do we get from here to there?