Re: [Qemu-devel] virtio block device and sysfs

[Marc Haber]

Hi,

On Mon, Mar 22, 2010 at 03:14:48PM +0000, Paul Brook wrote:
> > > John attempted this and it was reverted because the implementation
> > > exhausted the PCI config space.
> > 
> > I don't understand that. Existig hardware devices dump much more of
> > their data such as vendor and model type information into the config
> > space, how can using a field that real hardware uses exhaust the
> > config space?
> 
> I think you (collectively) are confusing three different things:

Perfectly accepted. I don't know zilch about today's hardware, I am
not a developer but only a systems administrator wishing for some more
flexibility in a software that I use every day.

> - PCI config space: Not used in any significant way other than the standard 
> fields (i.e. basic device ID and configuring BARs). Slow/hard to access.
> 
> - PCI IO BAR: Used by virtio-pci devices to expose the virtio config space. 
> Extremely limited resource (max. 64k for the whole system). On real hardware 
> only really used for legacy interfaces because it's slow, cumbersome, and x86 
> specific.  Unfortunately it currently seems to lower overhead than MMIO on 
> KVM 
> :-(
> 
> - PCI MEM BAR (usually MMIO): How real devices expose themselves. Relatively 
> large address space (megabytes) available.

Looks like PCI MEM BAR may be the way to go.

Greetings
Marc

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