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From: Arnd Bergmann <arnd@arndb.de>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: A set of "standard" virtual devices?
Date: Tue, 3 Apr 2007 23:51:16 +0200
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On Tuesday 03 April 2007, Jeremy Fitzhardinge wrote:
> That said, something like USB is probably the best bet for this kind of
> low-performance device.  I think.  Not that I really know anything about
> USB.

USB has the disadvantage that it is more complex than PCI and requires
significantly more code to simulate on the host side.

On the plus side, I think it should be possible to implement a virtual
USB host on s390, which is not possible with PCI, but that again takes
a lot of work to implement.

One interesting aspect of the PS3 hypervisor is that some of the
low-speed interfaces are implemented as a virtual UART, meaning
something that only has read and write operations and uses an
interrupt for flow control. The implementation in 
drivers/ps3/vuart.c is probably more complex than what we want
as a generic transport mechanism, but simply having a bidirectional
data stream sounds like an ideal abstraction for the "simple"
case. Some more or less obvious users of this include:

- console
- additional tty
- random
- slow network (using ppp)
- printer
- watchdog
- hid (e.g. mouse)
- system management (like ps3)
- fast network (in combination with
  shared memory segment)

The transport can be hypervisor specific, e.g. there could be
a virtual PCI serial port on kvm, an hcall interface on the ps3
and a virtual CTC on s390 (kidding), while all of them can have
the same kind of hardware _behind_ the serial connection.

	Arnd <><