linux-kernel - Re: [kvm-devel] [PATCH 3/3] virtio PCI device

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Date:	Tue, 27 Nov 2007 10:25:59 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	kvm-devel@...ts.sourceforge.net
Cc:	Anthony Liguori <aliguori@...ibm.com>,
	Eric Van Hensbergen <ericvanhensbergen@...ibm.com>,
	kvm-devel@...ts.sourceforge.net, lguest <lguest@...abs.org>,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org
Subject: Re: [kvm-devel] [PATCH 3/3] virtio PCI device

On Tuesday 27 November 2007, Avi Kivity wrote:
> > :-)  Do you know if there is a hard limit on the number of devices on 
> > a PCI bus?  My concern was that it was limited by something stupid 
> > like an 8-bit identifier.
> 
> IIRC pci slots are 8-bit, but you can have multiple buses, so 
> effectively 16 bits of device address space (discounting functions which 
> are likely not hot-pluggable).

You have an 8 bit bus number and an 8 bit device/function number.
The function number is 3 bits, so if you want to use only function 0
for everything, you are limited to a little under 8192 (2^(8+5)) devices
per PCI domain. PC style hardware cannot easily address multiple PCI
domains, but I think you can have them if you assume that the guest is
using mmconfig.

For using multiple buses, the easiest way could be to have every
device/function on bus 0 be a bridge by itself, so you end up with a
flat number space for the actual devices,

$ lspci -t
 [0000:00]-+-00.0-[0000:01]--+-00.0
           |                 +-01.0
           |                 +-02.0
           |                 + ...
           |                 \-3f.0
           +-00.1-[0000:02]--+-00.0
           |                 +-01.0
           |                 +-02.0
           |                 + ...
           |                 \-3f.0
           + ...
           |
           +-3f.6-[0000:ff]--+-00.0
                             +-01.0
                             +-02.0
                             + ...
                             \-3f.0

	Arnd <><
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