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

[Daniel P . Berrangé]

On Wed, Dec 19, 2018 at 11:50:38AM -0500, Michael S. Tsirkin wrote:
> On Wed, Dec 19, 2018 at 04:01:02PM +0000, Daniel P. Berrangé wrote:
> > On Wed, Dec 19, 2018 at 10:55:09AM -0500, Michael S. Tsirkin 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.
> > 
> > Well "nowait" is only referring to the chardev behaviour which is
> > still an accurate description.
> 
> man page seems to say
> 
>            nowait specifies that QEMU should not block waiting
>            for a client to connect to a listening socket.
> 
> if we do wait for a client to connect then how is it accurate?

Well the manpage needs to be clarified that this applies to the
initialization of the chardev.  What a downstream objet/device
does with the chardev is outside the scope of chardev config
options.

Regards,
Daniel
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