Re: [PATCH v11 1/3] iommu: Enhance IOMMU default DMA mode build options

From: John Garry
Date: Wed Jun 09 2021 - 11:30:10 EST

On 09/06/2021 16:03, Robin Murphy wrote:
On 2021-06-08 14:18, John Garry wrote:
From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>

First, add build options IOMMU_DEFAULT_{LAZY|STRICT}, so that we have the
opportunity to set {lazy|strict} mode as default at build time. Then put
the two config options in a choice, as they are mutually exclusive.

[jpg: Make choice between strict and lazy only (and not passthrough)]
Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
  drivers/iommu/Kconfig | 35 +++++++++++++++++++++++++++++++++++
  drivers/iommu/iommu.c |  3 ++-
  2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index 1f111b399bca..369a3af9e5bf 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -90,6 +90,41 @@ config IOMMU_DEFAULT_PASSTHROUGH
        If unsure, say N here.
+    prompt "IOMMU default DMA mode"
+    depends on IOMMU_API
+    depends on X86 || IA64 || X86_64 || ARM || ARM64

Simply "depends on IOMMU_DMA" should suffice, since that's now the only place where flush queues matter.

I suppose so.

Configs ARM64, AMD_IOMMU, and INTEL_IOMMU all select this.

+    help
+      This option allows an IOMMU DMA mode to be chosen at build time, to
+      override the default DMA mode of each ARCH, removing the need to
+      pass in kernel parameters through command line. It is still possible
+      to provide ARCH-specific or common boot options to override this
+      option.
+      If unsure, keep the default.
+    bool "lazy"
+    help
+      Support lazy mode, where for every IOMMU DMA unmap operation, the
+      flush operation of IOTLB and the free operation of IOVA are deferred.
+      They are only guaranteed to be done before the related IOVA will be
+      reused.
+    bool "strict"
+    help
+      For every IOMMU DMA unmap operation, the flush operation of IOTLB and
+      the free operation of IOVA are guaranteed to be done in the unmap
+      function.
+      This mode is safer than lazy mode, but it may be slower in some high
+      performance scenarios.

FWIW, as an end user who doesn't care much about the implementation details I'd probably appreciate the actual implications being clearer, i.e. what does "safer" mean in practice and what is it relative to?

Fine, I can mention that lazy mode means that we have reduced device isolation and a dangerous window can be created between device driver DMA unmap and zapping the mapping in the IOMMU; however still much safer than passthrough/no IOMMU, which means no isolation at all.