iommu: Clarify default domain Kconfig

Although strictly it is the AMD and Intel drivers which have an existing
expectation of lazy behaviour by default, it ends up being rather
unintuitive to describe this literally in Kconfig. Express it instead as
an architecture dependency, to clarify that it is a valid config-time
decision. The end result is the same since virtio-iommu doesn't support
lazy mode and thus falls back to strict at runtime regardless.

The per-architecture disparity is a matter of historical expectations:
the AMD and Intel drivers have been lazy by default since 2008, and
changing that gets noticed by people asking where their I/O throughput
has gone. Conversely, Arm-based systems with their wider assortment of
IOMMU drivers mostly only support strict mode anyway; only the Arm SMMU
drivers have later grown support for passthrough and lazy mode, for
users who wanted to explicitly trade off isolation for performance.
These days, reducing the default level of isolation in a way which may
go unnoticed by users who expect otherwise hardly seems worth risking
for the sake of one line of Kconfig, so here's where we are.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/69a0c6f17b000b54b8333ee42b3124c1d5a869e2.1631105737.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index f14f2e40107349de24fd5827d1e5f6d3d6e9ffea..4aa626cf00d880e1da317938ba95ab78f3c9fc12 100644 (file)
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -82,7 +82,7 @@ config IOMMU_DEBUGFS
 choice
        prompt "IOMMU default domain type"
        depends on IOMMU_API
-       default IOMMU_DEFAULT_DMA_LAZY if AMD_IOMMU || INTEL_IOMMU
+       default IOMMU_DEFAULT_DMA_LAZY if X86 || IA64
        default IOMMU_DEFAULT_DMA_STRICT
        help
          Choose the type of IOMMU domain used to manage DMA API usage by