[PATCH 0/2] iommupt: fix zero-length unmap causing a full IOTLB flush

[PATCH 0/2] iommupt: fix zero-length unmap causing a full IOTLB flush

[YeeLi]

From: yeeli <seven.yi.lee@gmail.com>

This series fixes an unmap that removes nothing from being turned into a
full-domain IOTLB flush, and adds a KUnit test for it.

Disclosure of tool use: the bug analysis and the initial drafts of both
patches were produced with the help of an AI coding assistant
(DeepSeek-V4.1-Flash, running in OpenCode). The prompts were a review of
the Intel IOMMU generic_pt integration, followed by requests to
reproduce, fix and test the empty unmap behaviour found there. The
assistant reviewed the iommupt unmap path and proposed the len guard in
gather_range_pending() plus the KUnit test; the code, commit messages
and test were then reviewed by the submitter. The submitter also
verified the behaviour independently: KUnit on the vtdss format, where
the new test fails without the fix and passes with it (full
vtdss_iommu_test: 63 tests, 60 passed, 3 skipped, 0 failed), and an
out-of-tree module on an Intel VT-d system observed with the
cache_tag_flush_range and qi_submit tracepoints. Everything that was
attempted could be built and tested. The submitter takes responsibility
for the submission.


yeeli (2):
  iommupt: fix full IOTLB flush from a zero length unmap
  iommupt: add a KUnit test for unmapping an empty range

 drivers/iommu/generic_pt/iommu_pt.h       |  7 ++++-
 drivers/iommu/generic_pt/kunit_iommu_pt.h | 31 +++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

-- 
2.34.1

[PATCH 1/2] iommupt: fix full IOTLB flush from a zero length unmap

[YeeLi]

From: yeeli <seven.yi.lee@gmail.com>

unmap_range() hands gather_range_pending() the number of bytes it
actually unmapped, not the requested length. That count is 0 whenever
the range holds no mappings, so an unmap that removes nothing reaches
the gather code with len == 0.

The gather helpers assume a real range: both
iommu_iotlb_gather_add_range() and iommu_iotlb_gather_is_disjoint()
compute end = iova + len - 1. For len == 0 this wraps to iova - 1.

The problem can be reproduced by attaching an empty paging domain to an
Intel VT-d device and unmapping IOVA 0:

	domain = iommu_paging_domain_alloc(dev);
	iommu_attach_device(domain, dev);
	iommu_unmap(domain, 0, PAGE_SIZE);	/* nothing is mapped */

The unmap returns 0, but the gather becomes [0, ULONG_MAX] and the
iotlb_sync() that follows asks the driver to invalidate everything.
Captured with the cache_tag_flush_range and qi_submit tracepoints
enabled:

  cache_tag_flush_range: dmar0 dmar0[0] type iotlb did 12 [0x0-0xffffffffffffffff] addr 0x0 mask 0x34
  qi_submit: p_iotlb_inv dmar0: 0xc0026 0x0 0x0 0x0

With a non-zero IOVA the wrapped end widens whatever range has already
been gathered instead.

Skip the gather bookkeeping when nothing was unmapped. An untouched
gather keeps start == ULONG_MAX and end == 0, which iommu_iotlb_sync()
already ignores.

Fixes: 7c53f4238aa8 ("iommupt: Add unmap_pages op")
Signed-off-by: yeeli <seven.yi.lee@gmail.com>
---
 drivers/iommu/generic_pt/iommu_pt.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/generic_pt/iommu_pt.h b/drivers/iommu/generic_pt/iommu_pt.h
index 837d835e4fad..566cae4bc217 100644
--- a/drivers/iommu/generic_pt/iommu_pt.h
+++ b/drivers/iommu/generic_pt/iommu_pt.h
@@ -83,8 +83,13 @@ static void gather_range_pending(struct iommupt_pending_gather *pending,
 	 * For NO_GAPS the user gets to pick if flushing all or doing micro
 	 * flushes is better for their work load by choosing DMA vs DMA-FQ
 	 * operation. Drivers should also see shadow_on_flush.
+	 *
+	 * A zero length unmap (len == 0) means nothing was unmapped and must
+	 * not create a gather range: iova + len - 1 wraps around, and at IOVA 0
+	 * the resulting degenerate range [0, ULONG_MAX] would turn into a full
+	 * IOTLB flush.
 	 */
-	if (!iommu_iotlb_gather_queued(iotlb_gather)) {
+	if (len && !iommu_iotlb_gather_queued(iotlb_gather)) {
 		if (pt_feature(common, PT_FEAT_FLUSH_RANGE_NO_GAPS) &&
 		    iommu_iotlb_gather_is_disjoint(iotlb_gather, iova, len)) {
 			iommu_iotlb_sync(&iommu_table->domain, iotlb_gather);
-- 
2.34.1

Re: [PATCH 1/2] iommupt: fix full IOTLB flush from a zero length unmap

[Baolu Lu]

On 11/5/2026 8:12 PM, YeeLi wrote:
> From: yeeli<seven.yi.lee@gmail.com>
> 
> unmap_range() hands gather_range_pending() the number of bytes it
> actually unmapped, not the requested length. That count is 0 whenever
> the range holds no mappings, so an unmap that removes nothing reaches
> the gather code with len == 0.
> 
> The gather helpers assume a real range: both
> iommu_iotlb_gather_add_range() and iommu_iotlb_gather_is_disjoint()
> compute end = iova + len - 1. For len == 0 this wraps to iova - 1.
> 
> The problem can be reproduced by attaching an empty paging domain to an
> Intel VT-d device and unmapping IOVA 0:
> 
> 	domain = iommu_paging_domain_alloc(dev);
> 	iommu_attach_device(domain, dev);
> 	iommu_unmap(domain, 0, PAGE_SIZE);	/* nothing is mapped */
> 
> The unmap returns 0, but the gather becomes [0, ULONG_MAX] and the
> iotlb_sync() that follows asks the driver to invalidate everything.
> Captured with the cache_tag_flush_range and qi_submit tracepoints
> enabled:
> 
>    cache_tag_flush_range: dmar0 dmar0[0] type iotlb did 12 [0x0-0xffffffffffffffff] addr 0x0 mask 0x34
>    qi_submit: p_iotlb_inv dmar0: 0xc0026 0x0 0x0 0x0
> 
> With a non-zero IOVA the wrapped end widens whatever range has already
> been gathered instead.
> 
> Skip the gather bookkeeping when nothing was unmapped. An untouched
> gather keeps start == ULONG_MAX and end == 0, which iommu_iotlb_sync()
> already ignores.
> 
> Fixes: 7c53f4238aa8 ("iommupt: Add unmap_pages op")
> Signed-off-by: yeeli<seven.yi.lee@gmail.com>
> ---
>   drivers/iommu/generic_pt/iommu_pt.h | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>

[PATCH 2/2] iommupt: add a KUnit test for unmapping an empty range

[YeeLi]

From: yeeli <seven.yi.lee@gmail.com>

Unmapping a range that holds no mappings must not record anything in
the IOTLB gather. Otherwise iova + len - 1 underflows and the flush
covers the whole domain, or widens a range that was already gathered.

Check that the gather stays empty for a hole at IOVA 0 and at a
non-zero IOVA. Without the previous fix the first case turns the gather
into [0, ULONG_MAX] and the second one flips it to [iova, iova - 1].

Signed-off-by: yeeli <seven.yi.lee@gmail.com>
---
 drivers/iommu/generic_pt/kunit_iommu_pt.h | 31 +++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/drivers/iommu/generic_pt/kunit_iommu_pt.h b/drivers/iommu/generic_pt/kunit_iommu_pt.h
index 9fc71db74234..59a9386f1474 100644
--- a/drivers/iommu/generic_pt/kunit_iommu_pt.h
+++ b/drivers/iommu/generic_pt/kunit_iommu_pt.h
@@ -427,6 +427,36 @@ static void test_mixed(struct kunit *test)
 	check_iova(test, start, oa, len);
 }
 
+/*
+ * Unmapping a range that holds no mappings must not record anything in
+ * the IOTLB gather. Otherwise iova + len - 1 underflows and the flush
+ * covers the whole domain, or widens a range that was already gathered.
+ */
+static void test_unmap_hole(struct kunit *test)
+{
+	struct kunit_iommu_priv *priv = test->priv;
+	struct iommu_iotlb_gather gather;
+
+	/* A hole at IOVA 0 must not turn the gather into [0, ULONG_MAX]. */
+	iommu_iotlb_gather_init(&gather);
+	KUNIT_ASSERT_EQ(
+		test,
+		iommu_unmap_fast(&priv->domain, 0, priv->smallest_pgsz, &gather),
+		0);
+	KUNIT_EXPECT_EQ(test, gather.start, ULONG_MAX);
+	KUNIT_EXPECT_EQ(test, gather.end, 0);
+
+	/* Neither may a hole at a non-zero IOVA widen the gather. */
+	iommu_iotlb_gather_init(&gather);
+	KUNIT_ASSERT_EQ(
+		test,
+		iommu_unmap_fast(&priv->domain, priv->smallest_pgsz,
+				 priv->smallest_pgsz, &gather),
+		0);
+	KUNIT_EXPECT_EQ(test, gather.start, ULONG_MAX);
+	KUNIT_EXPECT_EQ(test, gather.end, 0);
+}
+
 static struct kunit_case iommu_test_cases[] = {
 	KUNIT_CASE_FMT(test_increase_level),
 	KUNIT_CASE_FMT(test_map_simple),
@@ -435,6 +465,7 @@ static struct kunit_case iommu_test_cases[] = {
 	KUNIT_CASE_FMT(test_random_map),
 	KUNIT_CASE_FMT(test_pgsize_boundary),
 	KUNIT_CASE_FMT(test_mixed),
+	KUNIT_CASE_FMT(test_unmap_hole),
 	{},
 };
 
-- 
2.34.1