[PATCH v3 5/6] iommu/amd: Tailored gather logic for AMD

[Nadav Amit]

From: Nadav Amit <namit@xxxxxxxxxx>

AMD's IOMMU can flush efficiently (i.e., in a single flush) any range.
This is in contrast, for instnace, to Intel IOMMUs that have a limit on
the number of pages that can be flushed in a single flush.  In addition,
AMD's IOMMU do not care about the page-size, so changes of the page size
do not need to trigger a TLB flush.

So in most cases, a TLB flush due to disjoint range or page-size changes
are not needed for AMD. Yet, vIOMMUs require the hypervisor to
synchronize the virtualized IOMMU's PTEs with the physical ones. This
process induce overheads, so it is better not to cause unnecessary
flushes, i.e., flushes of PTEs that were not modified.

Implement and use amd_iommu_iotlb_gather_add_page() and use it instead
of the generic iommu_iotlb_gather_add_page(). Ignore page-size changes
and disjoint regions unless "non-present cache" feature is reported by
the IOMMU capabilities, as this is an indication we are running on a
physical IOMMU. A similar indication is used by VT-d (see "caching
mode"). The new logic retains the same flushing behavior that we had
before the introduction of page-selective IOTLB flushes for AMD.

On virtualized environments, check if the newly flushed region and the
gathered one are disjoint and flush if it is. Also check whether the new
region would cause IOTLB invalidation of large region that would include
unmodified PTE. The latter check is done according to the "order" of the
IOTLB flush.

Cc: Joerg Roedel <joro@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Jiajun Cao <caojiajun@xxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
 drivers/iommu/amd/iommu.c | 44 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 3e40f6610b6a..128f2e889ced 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -2053,6 +2053,48 @@ static int amd_iommu_map(struct iommu_domain *dom, unsigned long iova,
 	return ret;
 }
 
+static void amd_iommu_iotlb_gather_add_page(struct iommu_domain *domain,
+					    struct iommu_iotlb_gather *gather,
+					    unsigned long iova, size_t size)
+{
+	/*
+	 * AMD's IOMMU can flush as many pages as necessary in a single flush.
+	 * Unless we run in a virtual machine, which can be inferred according
+	 * to whether "non-present cache" is on, it is probably best to prefer
+	 * (potentially) too extensive TLB flushing (i.e., more misses) over
+	 * mutliple TLB flushes (i.e., more flushes). For virtual machines the
+	 * hypervisor needs to synchronize the host IOMMU PTEs with those of
+	 * the guest, and the trade-off is different: unnecessary TLB flushes
+	 * should be avoided.
+	 */
+	if (amd_iommu_np_cache && gather->end != 0) {
+		unsigned long start = iova, end = start + size - 1;
+
+		if (iommu_iotlb_gather_is_disjoint(gather, iova, size)) {
+			/*
+			 * If the new page is disjoint from the current range,
+			 * flush.
+			 */
+			iommu_iotlb_sync(domain, gather);
+		} else {
+			/*
+			 * If the order of TLB flushes increases by more than
+			 * 1, it means that we would have to flush PTEs that
+			 * were not modified. In this case, flush.
+			 */
+			unsigned long new_start = min(gather->start, start);
+			unsigned long new_end = min(gather->end, end);
+			int msb_diff = fls64(gather->end ^ gather->start);
+			int new_msb_diff = fls64(new_end ^ new_start);
+
+			if (new_msb_diff > msb_diff + 1)
+				iommu_iotlb_sync(domain, gather);
+		}
+	}
+
+	iommu_iotlb_gather_add_range(gather, iova, size);
+}
+
 static size_t amd_iommu_unmap(struct iommu_domain *dom, unsigned long iova,
 			      size_t page_size,
 			      struct iommu_iotlb_gather *gather)
@@ -2067,7 +2109,7 @@ static size_t amd_iommu_unmap(struct iommu_domain *dom, unsigned long iova,
 
 	r = (ops->unmap) ? ops->unmap(ops, iova, page_size, gather) : 0;
 
-	iommu_iotlb_gather_add_page(dom, gather, iova, page_size);
+	amd_iommu_iotlb_gather_add_page(dom, gather, iova, page_size);
 
 	return r;
 }
-- 
2.25.1