[PATCH] iommupt: report the whole contiguous group in read_and_clear_dirty

[samarth.colleges@gmail.com]

From: Samarth Narang <samarth.colleges@gmail.com>

The record_dirty() function computes the reported dirty length of a contiguous
(multi-item) leaf group as:

	end_index = log2_set_mod_max_t(unsigned int, pts->index, num_contig_lg2);
	end_index = min(end_index, pts->end_index);
	dirty_len = (end_index - index) * log2_to_int(pt_table_item_lg2sz(pts));

Here, log2_set_mod_max_t() sets the low num_contig_lg2 bits of the index to all
ones, i.e. it returns the *inclusive* last index of the contiguous group.
However, pts->end_index is an *exclusive* bound, this can be seen from the
definition of pt_range_to_end_index() which returns of the form "index + 1".
Also, the iterator stops on index >= end_index. Therefore, subtracting an
inclusive end from an exclusive bound will yield one less item than the actual
length of the contiguous group.

For IOMMU dirty tracking used by VFIO/iommufd live migration this silently
drops the last page of a contiguous dirty mapping from the migration
bitmap.

This patch makes end_index exclusive to match pts->end_index and the span cleared by
pt_entry_make_write_clean(). Also, the same length is passed to
iommu_iotlb_gather_add_range() so this fix would correct that as well.

Fixes: 4a00f9434891 ("iommupt: Add read_and_clear_dirty op")
Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>
---
 drivers/iommu/generic_pt/iommu_pt.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iommu/generic_pt/iommu_pt.h b/drivers/iommu/generic_pt/iommu_pt.h
--- a/drivers/iommu/generic_pt/iommu_pt.h
+++ b/drivers/iommu/generic_pt/iommu_pt.h
@@ -248,7 +248,7 @@ static void record_dirty(struct pt_state *pts,
 	if (num_contig_lg2 != ilog2(1)) {
 		unsigned int index = pts->index;
 		unsigned int end_index = log2_set_mod_max_t(
-			unsigned int, pts->index, num_contig_lg2);
+			unsigned int, pts->index, num_contig_lg2) + 1;
 
 		/* Adjust for being contained inside a contiguous page */
 		end_index = min(end_index, pts->end_index);