Re: [PATCH v2 1/1] virtio-gpu: get p2pdma distance

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 19, 2025 at 10:45:14AM +0000, Zhang, Julia wrote:
> >>> Can you explain more details about how to let
> >>> guest discover the way PCI driver discovers in physical world?
> >>>
> >> This is QEMU or VMM level implementation question on how to build the synthetic topology.
> >> In libvirt and other places, there is a way to describe the PCI level BDF etc details.
> >> You might want to check with the right owners on how to achieve or extend it.
> > 
> > Hi MST,
> > As discussed above, Parav suggested to rebuild PCI topology in QEMU
> > level to make virtual PCI topology 100% same as physical PCI topology so
> > that we can get accurate p2pdma_distance from guest directly. Since you
> > are one of the maintainers of QEMU, can you give us some suggestions or
> > explain more details about how to achieve it?
> > 
> > Thanks,
> > Julia
> 
> Hi MST,
> Since Parav thinks I should rebuild PCI topology in QEMU to make it 100% 
> same as physical PCI topology, which I think will be a big change for 
> virtual PCI. Can you give us some suggestions please?
> 
> Thanks,
> Julia

Hi Julia,
I feel the issue is not "100% matching" or the specific interface, but
that the motivation for the patches is not documented sufficiently
clearly.

It is true that many hypervisor vendors have a preference to utilizing native
interfaces, since this reduces the amount of guest code to maintain.
Remember, that guest code has to be implemented for every OS.
Further, duplicating e.g. ACPI information at the device level
means that baremetal and VM guest behaviour use different drivers,
which increases support and testing load.

I also note that the admin command infrastructure was designed with
use-cases where one device describes other devices.

Having said all that, it's not a religious argument, and might be reasonable
e.g. if the infrastructure is reusable to address a lot of other
problems.

Generally, one way to move things forward would be to show an example: 
- a baremetal instance which has the issue
- how is the distance discovered on baremetal
- how would a minimal emulation of baremetal look like
- why is this hard


Hope this helps,
-- 
MST