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

[Alex Williamson]

On Mon, 30 Mar 2026 12:56:51 -0300
Jason Gunthorpe <jgg@xxxxxxxx> wrote:

> On Mon, Mar 30, 2026 at 09:36:46AM -0600, Alex Williamson wrote:
> > There's risk involved with changing the default shift.  The fear is
> > there's userspace drivers that hard code the shift.  DPDK was even such
> > a user at one point, iirc.  Maybe it's ok to break such users, maybe
> > there are actually no such users left and it's all FUD at this point.
> > Either way, I have a hard time justifying that risk for a single,
> > obscure S390 device.  
> 
> If we go ahead with that DMABUF series could obscure cases like this
> be told to just get a DMABUF FD and then mmap it? Avoid this whole
> issue?

The ISM device here doesn't support mmap, so that much is still a
problem.  However, if we imagine a future where we've fully converted
to dma-buf for both DMA and CPU access to BARs, then maybe...  I don't
know how we'd describe a region overflow in the "legacy" ioctls to
maintain some consistency between what's exposed through the region
versus the dma-buf.  Thanks,

Alex