On Tue, Apr 14, 2020 at 09:25:58AM +0200, Markus Armbruster wrote: > 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?
Ignoring back compat, what would be our ideal CLI syntax ? Current syntax is -chardev socket,id=charnet1,path=/tmp/vhost1.sock -netdev vhost-user,chardev=charnet1,id=hostnet1 Should we have an option that expresses a "SocketAddress" struct on the CLI ? -socket type=unix,path=/tmp/vhost1.sock,id=sock0 -netdev vhost-user,socket=sock0,id=hostnet1 IIUC, Marc-André is suggesting that we carry on using -chardev, but detect when it is a socket chardev, and then ignore chardev APIs and create a QIOChannel. I can see some appeal in this as it provides a way to get all existing usage switched over, but I feel uneasy about sticking with -chardev forever, if we're not actually using a chardev. We could do the magic -chardev -> -socket conversion though, for a short period of time to ease the transition. We would have to 1. Introduce the new -socket and add "socket=$id" to devices that need it 2. Deprecate -chardev with type != socket, with no repacement intended 3. Deprecate -chardev with type == socket, translating to -socket ...wait 2 releases... 4. Delete support for "chardev=$id" from devices with "socket=$id" The hardest part is probably deciding exactly which set of devices can be restricted to only sockets, and which must have the full range of chardev backends available. 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 :|