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

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Date:	Fri, 11 Sep 2009 22:08:06 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH] virtio_console: Add support for multiple ports for
	generic guest and host communication

On (Fri) Sep 11 2009 [17:00:10], 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.

Right; there was some discussion about this. A few alternatives were
suggested like

- udev scripts to create symlinks from ports to function, like:

  /dev/vcon3 -> /dev/virtio-console/clipboard

- Some fqdn-like hierarchy, like

  /dev/virtio-console/com/redhat/clipboard

  which again can be created by udev scripts

> Anyway - you don't seem to need a fixed number you can use dynamic
> allocation and udev.
> 
> There are at least two better ways to do this
> 
> - Using sysfs nodes so you have a proper heirarchy of names/functions
> - Using a simple file system which provides a heirarchy of nodes whose
>   enumeration and access is backed by calls to whatever happyvisor you
>   are using.
> 
> it then self enumerates, self populates, doesn't need anyone to keep
> updating magic tables of guest code and expands cleanly - yes ?

Agreed. I'd prefer udev scripts doing it vs doing it in the code as it
keeps everything simple and the policy isn't laid out in the kernel
module. Is that fine?

		Amit
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