Re: [Qemu-devel] [PATCH v2 for-4.0 1/7] chardev: Add disconnected option for chardev socket

[Yongji Xie]

On Wed, 19 Dec 2018 at 23:55, Michael S. Tsirkin <address@hidden> wrote:
>
> On Tue, Dec 18, 2018 at 04:09:19PM +0000, Daniel P. Berrangé wrote:
> > On Tue, Dec 18, 2018 at 11:02:46AM -0500, Michael S. Tsirkin wrote:
> > > On Tue, Dec 18, 2018 at 03:25:20PM +0000, Daniel P. Berrangé wrote:
> > > > On Tue, Dec 18, 2018 at 04:24:26PM +0400, Marc-André Lureau wrote:
> > > > > Hi
> > > > >
> > > > > On Tue, Dec 18, 2018 at 2:01 PM <address@hidden> wrote:
> > > > > >
> > > > > > From: Xie Yongji <address@hidden>
> > > > > >
> > > > > > New option "disconnected" is added to init the chardev socket
> > > > > > in disconnected state. Then we can use qemu_chr_fe_wait_connected()
> > > > > > to connect when necessary. Now it would be used for unix domain
> > > > > > socket of vhost-user-blk device to support reconnect.
> > > > >
> > > > > What difference does that make if you wait for connection in
> > > > > qemu_chr_fe_wait_connected(), or during chardev setup?
> > > > >
> > > > > "disconnected" is misleading, would it be possible to reuse
> > > > > "wait/nowait" instead?
> > > >
> > > > Currently we default to doing a blocking connect in foreground,
> > > > except if reconnect is non-zero, in which case we do a connect
> > > > async in the background. This "disconnected" proposal effectively
> > > > does a blocking connect, but delayed to later in startup.
> > > >
> > > > IOW, this could already be achieved if "reconnect" were set to
> > > > non-zero. If the usage doesn't want reconnect though, I tend
> > > > to agree that we should use the exisiting wait/nowait options
> > > > to let it be controlled. I don't see that this "disconnected"
> > > > option gives a compelling benefit over using wait/nowait.
> > >
> > > So the tricky thing is that we can not expose the
> > > device to guest until we get a connection and can query
> > > it for the first time. After that is completed,
> > > we can reasonably support disconnected operation at least for net.
> >
> > The device code would still use  qemu_chr_fe_wait_connected() in my 
> > proposal,
> > so that its no different to the situation with the proposed "disconnected"
> > flag.
> >
> > Regards,
> > Daniel
>
> I guess so, but wouldn't it be confusing to users that one says
> "nowait" and qemu still waits for a connection and does not
> run the VM? That's different from how nowait behaves normally.
>

With this patch:

diff --git a/chardev/char-socket.c b/chardev/char-socket.c
index 2464d7a..ffe3a60 100644
--- a/chardev/char-socket.c
+++ b/chardev/char-socket.c
@@ -97,7 +97,10 @@ static void qemu_chr_socket_restart_timer(Chardev *chr)
     SocketChardev *s = SOCKET_CHARDEV(chr);
     char *name;

-    assert(s->connected == 0);
+    if (s->connected) {
+        return;
+    }
+
     name = g_strdup_printf("chardev-socket-reconnect-%s", chr->label);
     s->reconnect_timer = qemu_chr_timeout_add_ms(chr,
                                                  s->reconnect_time * 1000,

We can use qemu_chr_fe_wait_connected() without adding redundant
"wait" or "disconnected" option.
The disadavantage is that backend may have two connection from qemu
during initialization. But I think backend should support this case.

Thanks,
Yongji