linux-kernel - Re: [PATCH] virtio_console: Add support for multiple ports for generic guest and host communication

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Date:	Tue, 22 Sep 2009 21:15:49 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Alan Cox <alan@...ux.intel.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_console: Add support for multiple ports for
	generic guest and host communication

On (Tue) Sep 22 2009 [12:14:04], Rusty Russell wrote:
> On Sat, 12 Sep 2009 01:30:10 am Alan Cox wrote:
> > > The interface presented to guest userspace is of a simple char
> > > device, so it can be used like this:
> > > 
> > >     fd = open("/dev/vcon2", O_RDWR);
> > >     ret = read(fd, buf, 100);
> > >     ret = write(fd, string, strlen(string));
> > > 
> > > Each port is to be assigned a unique function, for example, the
> > > first 4 ports may be reserved for libvirt usage, the next 4 for
> > > generic streaming data and so on. This port-function mapping
> > > isn't finalised yet.
> > 
> > Unless I am missing something this looks completely bonkers
> > 
> > Every time we have a table of numbers for functionality it ends in
> > tears. We have to keep tables up to date and managed, we have to
> > administer the magical number to name space.
> 
> The number comes from the ABI; we need some identifier for the different
> ports.  Amit started using names, and I said "just use numbers"; they have
> to be documented and agreed by all clients anyway.
> 
> ie. the host says "here's a port id 7", which might be the cut & paste
> port or whatever.

Yeah; port 0 has to be reserved for a console (and then we might need
to do a bit more for multiple consoles -- hvc operates on a 'vtermno',
so we need to allocate them as well).

Also, a 'name' property can be attached to ports, as has been suggested:

qemu ... -device virtconport,name=org.qemu.clipboard,port=3,...

spawns a port at id 3 and the guest will also place a file:

/sys/class/virtio-console/vcon3/name

which has "org.qemu.clipboard" as contents, so udev scripts could
create a symlink:

/dev/vcon/org.qemu.clipboard -> /dev/vcon3

		Amit
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