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

Peter Xu
2025-06-17T23:26:32Z

On Tue, Jun 17, 2025 at 08:00:30PM -0300, Jason Gunthorpe wrote:
> On Tue, Jun 17, 2025 at 04:01:11PM -0400, Peter Xu wrote:
>
> > > 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.
>
> OK, CONFIG_MMU makes a little bit of sense
>
> > 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..
>
> That can't be fixed with a config, the logic in vfio_device_fops has
> to be
>
> if (!device->ops->get_unmapped_area()
> return .. do_default thing..
>
> return device->ops->get_unmapped()
>
> Has nothing to do with CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP, there are
> more device->ops that just PCI.

IMHO CONFIG_ARCH_SUPPORTS_HUGE_PFNMAP doesn't imply anything PCI specific
either, it only says an arch supports PFNMAP in larger than PAGE_SIZE.
IIUC it doesn't necessarily need to be PCI.

So here in this case, get_unmapped_area() will only be customized if the
kernel is compiled with any possible huge mapping on pfnmaps. Otherwise
the customized hook isn't needed.

>
> If you do the API with an align/order argument then the default
> behavior should happen when passing PAGE_SIZE.

This should indeed also work.

I'll wait for comments in the other threads. So far I didn't yet add the
"order" parameter or anything like it. If we would like to have the
parameter, I can use it here to avoid the ifdef with PAGE_SIZE / PAGE_SHIFT
/ .... when repost.

Said that, I don't think I understand at all the use of get_unmapped_area()
for !MMU use case.

Thanks,

--
Peter Xu