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

[Daniel P . Berrangé]

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
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