[PATCH v4 5/7] iommu/amd: Tailored gather logic for AMD

From: Nadav Amit
Date: Wed Jun 16 2021 - 13:34:44 EST

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 is 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 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.

Cc: Joerg Roedel <joro@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Jiajun Cao <caojiajun@xxxxxxxxxx>
Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx>
Cc: Robin Murphy <robin.murphy@xxxxxxx>
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
drivers/iommu/amd/iommu.c | 23 ++++++++++++++++++++++-
1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
index 3e40f6610b6a..63048aabaf5d 100644
--- a/drivers/iommu/amd/iommu.c
+++ b/drivers/iommu/amd/iommu.c
@@ -2053,6 +2053,27 @@ 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 &&
+ iommu_iotlb_gather_is_disjoint(gather, iova, size))
+ 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 +2088,7 @@ 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;