[PATCH 4/4] iommu/amd: Do not use flush-queue when NpCache is on

From: Nadav Amit
Date: Sun May 02 2021 - 03:05:33 EST


From: Nadav Amit <namit@xxxxxxxxxx>

Do not use flush-queue on virtualized environments, where the NpCache
capability of the IOMMU is set. This is required to reduce
virtualization overheads.

This change follows a similar change to Intel's VT-d and a detailed
explanation as for the rationale is described in commit 29b32839725f
("iommu/vt-d: Do not use flush-queue when caching-mode is on").

Cc: Joerg Roedel <joro@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Jiajun Cao <caojiajun@xxxxxxxxxx>
Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
drivers/iommu/amd/init.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index d006724f4dc2..ba3b76ed776d 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -1850,8 +1850,13 @@ static int __init iommu_init_pci(struct amd_iommu *iommu)
if (ret)
return ret;

- if (iommu->cap & (1UL << IOMMU_CAP_NPCACHE))
+ if (iommu->cap & (1UL << IOMMU_CAP_NPCACHE)) {
+ if (!amd_iommu_unmap_flush)
+ pr_warn_once("IOMMU batching is disabled due to virtualization");
+
amd_iommu_np_cache = true;
+ amd_iommu_unmap_flush = true;
+ }

init_iommu_perf_ctr(iommu);

--
2.25.1