[PATCH v2 1/1] vfio/type1: Respect IOMMU reserved regions in vfio_test_domain_fgsp()

[Niklas Schnelle]

Since commit cbf7827bc5dc ("iommu/s390: Fix potential s390_domain
aperture shrinking") the s390 IOMMU driver uses reserved regions for the
system provided DMA ranges of PCI devices. Previously it reduced the
size of the IOMMU aperture and checked it on each mapping operation.
On current machines the system denies use of DMA addresses below 2^32 for
all PCI devices.

Usually mapping IOVAs in a reserved regions is harmless until a DMA
actually tries to utilize the mapping. However on s390 there is
a virtual PCI device called ISM which is implemented in firmware and
used for cross LPAR communication. Unlike real PCI devices this device
does not use the hardware IOMMU but inspects IOMMU translation tables
directly on IOTLB flush (s390 RPCIT instruction). If it detects IOVA
mappings outside the allowed ranges it goes into an error state. This
error state then causes the device to be unavailable to the KVM guest.

Analysing this we found that vfio_test_domain_fgsp() maps 2 pages at DMA
address 0 irrespective of the IOMMUs reserved regions. Even if usually
harmless this seems wrong in the general case so instead go through the
freshly updated IOVA list and try to find a range that isn't reserved,
and fits 2 pages, is PAGE_SIZE * 2 aligned. If found use that for
testing for fine grained super pages.

Fixes: 6fe1010d6d9c ("vfio/type1: DMA unmap chunking")
Reported-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
---
v1 -> v2:
- Reworded commit message to hopefully explain things a bit better and
  highlight that usually just mapping but not issuing DMAs for IOVAs in
  a resverved region is harmless but still breaks things with ISM devices.
- Added a check for PAGE_SIZE * 2 alignment (Jason)

 drivers/vfio/vfio_iommu_type1.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 23c24fe98c00..87b27ffb93d0 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -1856,24 +1856,32 @@ static int vfio_iommu_replay(struct vfio_iommu *iommu,
  * significantly boosts non-hugetlbfs mappings and doesn't seem to hurt when
  * hugetlbfs is in use.
  */
-static void vfio_test_domain_fgsp(struct vfio_domain *domain)
+static void vfio_test_domain_fgsp(struct vfio_domain *domain, struct list_head *regions)
 {
-	struct page *pages;
 	int ret, order = get_order(PAGE_SIZE * 2);
+	struct vfio_iova *region;
+	struct page *pages;
 
 	pages = alloc_pages(GFP_KERNEL | __GFP_ZERO, order);
 	if (!pages)
 		return;
 
-	ret = iommu_map(domain->domain, 0, page_to_phys(pages), PAGE_SIZE * 2,
-			IOMMU_READ | IOMMU_WRITE | IOMMU_CACHE);
-	if (!ret) {
-		size_t unmapped = iommu_unmap(domain->domain, 0, PAGE_SIZE);
+	list_for_each_entry(region, regions, list) {
+		if (region->end - region->start < PAGE_SIZE * 2 ||
+				region->start % (PAGE_SIZE*2))
+			continue;
 
-		if (unmapped == PAGE_SIZE)
-			iommu_unmap(domain->domain, PAGE_SIZE, PAGE_SIZE);
-		else
-			domain->fgsp = true;
+		ret = iommu_map(domain->domain, region->start, page_to_phys(pages), PAGE_SIZE * 2,
+				IOMMU_READ | IOMMU_WRITE | IOMMU_CACHE);
+		if (!ret) {
+			size_t unmapped = iommu_unmap(domain->domain, region->start, PAGE_SIZE);
+
+			if (unmapped == PAGE_SIZE)
+				iommu_unmap(domain->domain, region->start + PAGE_SIZE, PAGE_SIZE);
+			else
+				domain->fgsp = true;
+		}
+		break;
 	}
 
 	__free_pages(pages, order);
@@ -2326,7 +2334,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
 		}
 	}
 
-	vfio_test_domain_fgsp(domain);
+	vfio_test_domain_fgsp(domain, &iova_copy);
 
 	/* replay mappings on new domains */
 	ret = vfio_iommu_replay(iommu, domain);
-- 
2.34.1