vfio: Add an IOVA bitmap support

The new facility adds a bunch of wrappers that abstract how an IOVA range
is represented in a bitmap that is granulated by a given page_size. So it
translates all the lifting of dealing with user pointers into its
corresponding kernel addresses backing said user memory into doing finally
the (non-atomic) bitmap ops to change various bits.

The formula for the bitmap is:

   data[(iova / page_size) / 64] & (1ULL << (iova % 64))

Where 64 is the number of bits in a unsigned long (depending on arch)

It introduces an IOVA iterator that uses a windowing scheme to minimize the
pinning overhead, as opposed to pinning it on demand 4K at a time. Assuming
a 4K kernel page and 4K requested page size, we can use a single kernel
page to hold 512 page pointers, mapping 2M of bitmap, representing 64G of
IOVA space.

An example usage of these helpers for a given @base_iova, @page_size,
@Length and __user @DaTa:

   bitmap = iova_bitmap_alloc(base_iova, page_size, length, data);
   if (IS_ERR(bitmap))
       return -ENOMEM;

   ret = iova_bitmap_for_each(bitmap, arg, dirty_reporter_fn);

   iova_bitmap_free(bitmap);

An implementation of the lower end -- referred to above as
dirty_reporter_fn to exemplify -- that is tracking dirty bits would mark
an IOVA as dirty as following:

	iova_bitmap_set(bitmap, iova, page_size);

Or a contiguous range (example two pages):

	iova_bitmap_set(bitmap, iova, 2 * page_size);

The facility is intended to be used for user bitmaps representing dirtied
IOVAs by IOMMU (via IOMMUFD) and PCI Devices (via vfio-pci).

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>

drivers/vfio/Makefile

@@ -1,9 +1,11 @@
 # SPDX-License-Identifier: GPL-2.0
 vfio_virqfd-y := virqfd.o
 
-vfio-y += vfio_main.o
-
 obj-$(CONFIG_VFIO) += vfio.o
+
+vfio-y += vfio_main.o \
+	  iova_bitmap.o \
+
 obj-$(CONFIG_VFIO_VIRQFD) += vfio_virqfd.o
 obj-$(CONFIG_VFIO_IOMMU_TYPE1) += vfio_iommu_type1.o
 obj-$(CONFIG_VFIO_IOMMU_SPAPR_TCE) += vfio_iommu_spapr_tce.o