Re: [PATCH 4/5] vfio: Introduce vfio_device_ops.get_unmapped_area hook

Peter Xu
2025-06-17T20:01:21Z

On Tue, Jun 17, 2025 at 04:46:21PM -0300, Jason Gunthorpe wrote:
> > I just noticed this is unfortunate and special; I yet don't see a way to
> > avoid the fallback here.
> >
> > Note that this is the vfio_device's fallback, even if the new helper
> > (whatever we name it..) could do fallback internally, vfio_device still
> > would need to be accessible to mm_get_unmapped_area() to make this config
> > build pass.
>
> I don't understand this remark?
>
> get_unmapped_area is not conditional on CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP?
>
> Some new mm_get_unmapped_area_aligned() should not be conditional on
> CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP? (This is Lorenzo's and Liam's remark)

Yes, this will be addressed.

>
> So what is VFIO doing that requires CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP?

It's the fallback part for vfio device, not vfio_pci device. vfio_pci
device doesn't need this special treatment after moving to the new helper
because that hides everything. vfio_device still needs it.

So, we have two ops that need to be touched to support this:

vfio_device_fops
vfio_pci_ops

For the 1st one's vfio_device_fops.get_unmapped_area(), it'll need its own
fallback which must be mm_get_unmapped_area() to keep the old behavior, and
that was defined only if CONFIG_MMU.

IOW, if one day file_operations.get_unmapped_area() would allow some other
retval to be able to fallback to the default (mm_get_unmapped_area()), then
we don't need this special ifdef. But now it's not ready for that..

--
Peter Xu