Re: virtio PCI on KVM without IO BARs

["H. Peter Anvin" <hpa@zytor.com>]

On 02/28/2013 07:24 AM, Michael S. Tsirkin wrote:
> 
> 3. hypervisor assigned IO address
> 	qemu can reserve IO addresses and assign to virtio devices.
> 	2 bytes per device (for notification and ISR access) will be
> 	enough. So we can reserve 4K and this gets us 2000 devices.
>         From KVM perspective, nothing changes.
> 	We'll want some capability in the device to let guest know
> 	this is what it should do, and pass the io address.
> 	One way to reserve the addresses is by using the bridge.
> 	Pros: no need for host kernel support
> 	Pros: regular PIO so fast
> 	Cons: does not help assigned devices, breaks nested virt
> 
> Simply counting pros/cons, option 3 seems best. It's also the
> easiest to implement.
> 

The problem here is the 4K I/O window for IO device BARs in bridges.
Why not simply add a (possibly proprietary) capability to the PCI bridge
to allow a much narrower window?  That fits much more nicely into the
device resource assignment on the guest side, and could even be
implemented on a real hardware device -- we can offer it to the PCI-SIG
for standardization, even.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.