Re: [RFT][PATCH v1 6/6] vfio: Replace phys_pfn with phys_page for vfio_pin_pages()

Christoph Hellwig
2022-06-20T06:38:01Z

On Sun, Jun 19, 2022 at 10:51:47PM -0700, Christoph Hellwig wrote:
> On Mon, Jun 20, 2022 at 12:00:46AM -0300, Jason Gunthorpe wrote:
> > On Fri, Jun 17, 2022 at 01:54:05AM -0700, Christoph Hellwig wrote:
> > > There is a bunch of code an comments in the iommu type1 code that
> > > suggest we can pin memory that is not page backed.
> >
> > AFAIK you can.. The whole follow_pte() mechanism allows raw PFNs to be
> > loaded into the type1 maps and the pin API will happily return
> > them. This happens in almost every qemu scenario because PCI MMIO BAR
> > memory ends up routed down this path.
>
> Indeed, my read wasn't deep enough. Which means that we can't change
> the ->pin_pages interface to return a struct pages array, as we don't
> have one for those.

Actually. gvt requires a struct page, and both s390 seem to require
normal non-I/O, non-remapped kernel pointers. So I think for the
vfio_pin_pages we can assume that we only want page backed memory and
remove the follow_fault_pfn case entirely. But we'll probably have
to keep it for the vfio_iommu_replay case that is not tied to
emulated IOMMU drivers.