[PATCH v4 23/24] iommu: Merge strictness and domain type configs

From: Robin Murphy
Date: Wed Aug 11 2021 - 08:23:19 EST

To parallel the sysfs behaviour, merge the new build-time option
for DMA domain strictness into the default domain type choice.

Suggested-by: Joerg Roedel <joro@xxxxxxxxxx>
Reviewed-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
Reviewed-by: John Garry <john.garry@xxxxxxxxxx>
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>

---

v3: Remember to update parameter documentation as well
---
.../admin-guide/kernel-parameters.txt | 8 +-
drivers/iommu/Kconfig | 80 +++++++++----------
drivers/iommu/iommu.c | 2 +-
3 files changed, 44 insertions(+), 46 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 90b525cf0ec2..19192b39952a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2045,11 +2045,9 @@
1 - Strict mode.
DMA unmap operations invalidate IOMMU hardware TLBs
synchronously.
- unset - Use value of CONFIG_IOMMU_DEFAULT_{LAZY,STRICT}.
- Note: on x86, the default behaviour depends on the
- equivalent driver-specific parameters, but a strict
- mode explicitly specified by either method takes
- precedence.
+ unset - Use value of CONFIG_IOMMU_DEFAULT_DMA_{LAZY,STRICT}.
+ Note: on x86, strict mode specified via one of the
+ legacy driver-specific options takes precedence.

iommu.passthrough=
[ARM64, X86] Configure DMA to bypass the IOMMU by default.
diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index c84da8205be7..6e06f876d75a 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -79,55 +79,55 @@ config IOMMU_DEBUGFS
debug/iommu directory, and then populate a subdirectory with
entries as required.

-config IOMMU_DEFAULT_PASSTHROUGH
- bool "IOMMU passthrough by default"
- depends on IOMMU_API
- help
- Enable passthrough by default, removing the need to pass in
- iommu.passthrough=on or iommu=pt through command line. If this
- is enabled, you can still disable with iommu.passthrough=off
- or iommu=nopt depending on the architecture.
-
- If unsure, say N here.
-
choice
- prompt "IOMMU default DMA IOTLB invalidation mode"
- depends on IOMMU_DMA
-
- default IOMMU_DEFAULT_LAZY if (AMD_IOMMU || INTEL_IOMMU)
- default IOMMU_DEFAULT_STRICT
+ prompt "IOMMU default domain type"
+ depends on IOMMU_API
+ default IOMMU_DEFAULT_DMA_LAZY if AMD_IOMMU || INTEL_IOMMU
+ default IOMMU_DEFAULT_DMA_STRICT
help
- This option allows an IOMMU DMA IOTLB invalidation mode to be
- chosen at build time, to override the default mode of each ARCH,
- removing the need to pass in kernel parameters through command line.
- It is still possible to provide common boot params to override this
- config.
+ Choose the type of IOMMU domain used to manage DMA API usage by
+ device drivers. The options here typically represent different
+ levels of tradeoff between robustness/security and performance,
+ depending on the IOMMU driver. Not all IOMMUs support all options.
+ This choice can be overridden at boot via the command line, and for
+ some devices also at runtime via sysfs.

If unsure, keep the default.

-config IOMMU_DEFAULT_STRICT
- bool "strict"
+config IOMMU_DEFAULT_DMA_STRICT
+ bool "Translated - 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.
+ Trusted devices use translation to restrict their access to only
+ DMA-mapped pages, with strict TLB invalidation on unmap. Equivalent
+ to passing "iommu.passthrough=0 iommu.strict=1" on the command line.

-config IOMMU_DEFAULT_LAZY
- bool "lazy"
+ Untrusted devices always use this mode, with an additional layer of
+ bounce-buffering such that they cannot gain access to any unrelated
+ data within a mapped page.
+
+config IOMMU_DEFAULT_DMA_LAZY
+ bool "Translated - 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.
+ Trusted devices use translation to restrict their access to only
+ DMA-mapped pages, but with "lazy" batched TLB invalidation. This
+ mode allows higher performance with some IOMMUs due to reduced TLB
+ flushing, but at the cost of reduced isolation since devices may be
+ able to access memory for some time after it has been unmapped.
+ Equivalent to passing "iommu.passthrough=0 iommu.strict=0" on the
+ command line.

- The isolation provided in this mode is not as secure as STRICT mode,
- such that a vulnerable time window may be created between the DMA
- unmap and the mappings cached in the IOMMU IOTLB or device TLB
- finally being invalidated, where the device could still access the
- memory which has already been unmapped by the device driver.
- However this mode may provide better performance in high throughput
- scenarios, and is still considerably more secure than passthrough
- mode or no IOMMU.
+ If this mode is not supported by the IOMMU driver, the effective
+ runtime default will fall back to IOMMU_DEFAULT_DMA_STRICT.
+
+config IOMMU_DEFAULT_PASSTHROUGH
+ bool "Passthrough"
+ help
+ Trusted devices are identity-mapped, giving them unrestricted access
+ to memory with minimal performance overhead. Equivalent to passing
+ "iommu.passthrough=1" (historically "iommu=pt") on the command line.
+
+ If this mode is not supported by the IOMMU driver, the effective
+ runtime default will fall back to IOMMU_DEFAULT_DMA_STRICT.

endchoice

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 63face36fc49..480ad6a538a9 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -31,7 +31,7 @@ static struct kset *iommu_group_kset;
static DEFINE_IDA(iommu_group_ida);

static unsigned int iommu_def_domain_type __read_mostly;
-static bool iommu_dma_strict __read_mostly = IS_ENABLED(CONFIG_IOMMU_DEFAULT_STRICT);
+static bool iommu_dma_strict __read_mostly = IS_ENABLED(CONFIG_IOMMU_DEFAULT_DMA_STRICT);
static u32 iommu_cmd_line __read_mostly;

struct iommu_group {
--
2.25.1