Re: virtio PCI on KVM without IO BARs

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

On 03/05/2013 04:05 PM, H. Peter Anvin wrote:
> 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.
> 

Just a correction: I'm of course not talking about BARs but of the
bridge windows.  The BARs are not a problem; an I/O BAR can cover as
little as four bytes.

	-hpa

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