linux-kernel - Re: A set of "standard" virtual devices?

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Date:	Tue, 03 Apr 2007 15:10:18 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Andi Kleen <ak@...e.de>,
	Christian Borntraeger <borntrae@...ibm.com>,
	virtualization@...ts.linux-foundation.org,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	mathiasen@...il.com
Subject: Re: A set of "standard" virtual devices?

Arnd Bergmann wrote:
> 
> 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.
> 

Note that at least for PIO-based devices, there is nothing that says you 
can't implement PCI over another transport, if you wish.  It's really 
just a very simple RPC protocol.

DMA is trickier, as it makes the data appear into the address space of 
the guest in a way that is both device- and host-dependent (in the 
presence of PCI domains, IOMMU etc.)  There may be reason to avoid DMA 
for that reason.

	-hpa
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