vfio/iova_bitmap: Fix PAGE_SIZE unaligned bitmaps

iova_bitmap_set() doesn't consider the end of the page boundary when the
first bitmap page offset isn't zero, and wrongly changes the consecutive
page right after. Consequently this leads to missing dirty pages from
reported by the device as seen from the VMM.

The current logic iterates over a given number of base pages and clamps it
to the remaining indexes to iterate in the last page.  Instead of having to
consider extra pages to pin (e.g. first and extra pages), just handle the
first page as its own range and let the rest of the bitmap be handled as if
it was base page aligned.

This is done by changing iova_bitmap_mapped_remaining() to return PAGE_SIZE
- pgoff (on the first bitmap page), and leads to pgoff being set to 0 on
following iterations.

Fixes: 58ccf019 ("vfio: Add an IOVA bitmap support")
Reported-by: Avihai Horon <avihaih@nvidia.com>
Tested-by: Avihai Horon <avihaih@nvidia.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20221025193114.58695-3-joao.m.martins@oracle.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/iova_bitmap.c

@@ -296,11 +296,15 @@ void iova_bitmap_free(struct iova_bitmap *bitmap)
  */
 static unsigned long iova_bitmap_mapped_remaining(struct iova_bitmap *bitmap)
 {
-	unsigned long remaining;
+	unsigned long remaining, bytes;
+
+	/* Cap to one page in the first iteration, if PAGE_SIZE unaligned. */
+	bytes = !bitmap->mapped.pgoff ? bitmap->mapped.npages << PAGE_SHIFT :
+					PAGE_SIZE - bitmap->mapped.pgoff;
 
 	remaining = bitmap->mapped_total_index - bitmap->mapped_base_index;
 	remaining = min_t(unsigned long, remaining,
-	      (bitmap->mapped.npages << PAGE_SHIFT) / sizeof(*bitmap->bitmap));
+			  bytes / sizeof(*bitmap->bitmap));
 
 	return remaining;
 }