Re: [PATCH v8 2/3] vfio/ism: Implement vfio_pci driver for ISM devices

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Mon, 2026-03-30 at 21:03 -0300, Jason Gunthorpe wrote:
> On Mon, Mar 30, 2026 at 12:39:25PM -0600, Alex Williamson wrote:
> > Yes, that would be the ideal balance of enabling larger BARs while
> > retaining compatibility for existing devices.  Sounds like we'll
> > continue down the variant driver path for this one-off device for
> > now.  It remains fairly self contained so long as we don't add more
> > regions for vfio-pci-core to manage.
> > 
> > This could be a good project if someone is looking though, especially
> > if it had a build-time config option to set the minimum region size so
> > we can exercise it before we hit 1TB BARs.  Thanks,
> 
> Well, it took claude an hour to vibe code a draft from a four sentence
> prompt.. I see many things that need changing/cleaning in here, but
> the patch strategy and general idea looks sound.
> 
> I guess another day of effort would probably get things into something
> presentable. We are not so far away - maybe the variant driver path is
> not necessary?
> 
> https://github.com/jgunthorpe/linux/commits/vfio_maple_tree/
> 
> Jason

I'm in no way against changing the offset handling in general but for
what it's worth, we need a variant driver for ISM regardless, though it
would be smaller with large offset support. This is because besides the
large BAR we also have to use our classic function handle based PCI
instructions to access that BAR instead of the newer memory-I/O (MIO)
PCI instructions. And sadly the way the ISM device uses the PCI Store
Block instruction there is no straight forward way to enable the newer
instructions even if we changed the device implementation. This also
means that we would want to keep the no-mmap() restriction.

Thanks,
Niklas