[PATCH v13 3/6] iommu: Enhance IOMMU default DMA mode build options

From: John Garry
Date: Wed Jun 16 2021 - 07:08:58 EST

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 an 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>
Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>
.../admin-guide/kernel-parameters.txt | 3 +-
drivers/iommu/Kconfig | 40 +++++++++++++++++++
drivers/iommu/iommu.c | 2 +-
3 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fcbb36d6eea7..d8fb36363be0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2052,9 +2052,10 @@
throughput at the cost of reduced device isolation.
Will fall back to strict mode if not supported by
the relevant IOMMU driver.
- 1 - Strict mode (default).
+ 1 - Strict mode.
DMA unmap operations invalidate IOMMU hardware TLBs
+ 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
diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index 1f111b399bca..0327a942fdb7 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -90,6 +90,46 @@ config IOMMU_DEFAULT_PASSTHROUGH

If unsure, say N here.

+ prompt "IOMMU default DMA IOTLB invalidation mode"
+ depends on IOMMU_DMA
+ 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.
+ If unsure, keep the default.
+ 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.
+ 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.
+ 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.
config OF_IOMMU
def_bool y
depends on OF && IOMMU_API
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index cf58949cc2f3..60b1ec42e73b 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -29,7 +29,7 @@ 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 = true;
+static bool iommu_dma_strict __read_mostly = IS_ENABLED(CONFIG_IOMMU_DEFAULT_STRICT);
static u32 iommu_cmd_line __read_mostly;

struct iommu_group {