Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR

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

On 06/05/2013 02:50 PM, Anthony Liguori wrote:
> "H. Peter Anvin" <hpa@zytor.com> writes:
> 
>> On 06/05/2013 09:20 AM, Michael S. Tsirkin wrote:
>>>
>>> Spec says IO and memory can be enabled/disabled, separately.
>>> PCI Express spec says devices should work without IO.
>>>
>>
>> For "native endpoints".  Currently virtio would be a "legacy endpoint"
>> which is quite correct -- it is compatible with a legacy interface.
> 
> Do legacy endpoints also use 4k for BARs?

There are no 4K BARs.  In fact, I/O BARs are restricted by spec (there
is no technical enforcement, however) to 256 bytes.

The 4K come from the upstream bridge windows, which are only 4K granular
(historic stuff from when bridges were assumed rare.)  However, there
can be multiple devices, functions, and BARs inside that window.

The issue with PCIe is that each PCIe port is a bridge, so in reality
there is only one real device per bus number.

> If not, can't we use a new device id for native endpoints and call it a
> day?  Legacy endpoints would continue using the existing BAR layout.

Definitely an option.  However, we want to be able to boot from native
devices, too, so having an I/O BAR (which would not be used by the OS
driver) should still at the very least be an option.

	-hpa