Re: [PATCH v2 1/1] iommu: Add config option to set lazy mode as default

From: Robin Murphy
Date: Wed Mar 27 2019 - 15:19:00 EST

On 27/03/2019 15:00, Zhen Lei wrote:
This allows the default behaviour to be controlled by a kernel config
option instead of changing the command line for the kernel to include
"iommu.strict=0" on ARM64 where this is desired.


Note: At present, intel_iommu, amd_iommu and s390_iommu use lazy mode as
default, so there is no need to add code for them.

That seems a bit self-contradictory - if there's a real need for TLB flush behaviour to be statically configurable and not command-line-based then why should users of other architectures be exempt?

Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
drivers/iommu/Kconfig | 14 ++++++++++++++
drivers/iommu/iommu.c | 5 +++++
2 files changed, 19 insertions(+)

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index 6f07f3b21816c64..5daa110d0e83a07 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -85,6 +85,20 @@ config IOMMU_DEFAULT_PASSTHROUGH

If unsure, say N here.

+ bool "IOMMU DMA use lazy mode to flush IOTLB and free IOVA"
+ depends on IOMMU_API
+ 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. Removing the need to pass in kernel parameters through
+ command line. For example, iommu.strict=0 on ARM64. If this is
+ enabled, you can still disable with kernel parameters, such as
+ iommu.strict=1 depending on the architecture.
+ If unsure, say N here.
config OF_IOMMU
def_bool y
depends on OF && IOMMU_API
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 33a982e33716369..5acb98e79b5b32d 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -43,7 +43,12 @@
static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_DMA;
+static bool iommu_dma_strict __read_mostly;
static bool iommu_dma_strict __read_mostly = true;

For a straightforward boolean, you can simply do:

static bool foo = IS_ENABLED(CONFIG_FOO);

but that said, I'm still not particularly convinced that there are a significant number of users in a position to build and install a custom kernel but not edit /etc/default/grub, and who really value the combination of less performance than passthrough with less isolation than strict.

It's also not necessarily obvious to the user how this interacts with IOMMU_DEFAULT_PASSTHROUGH, so if we really do go down this route, maybe it would be better to refactor the whole lot into a single selection of something like IOMMU_DEFAULT_MODE anyway.


struct iommu_callback_data {
const struct iommu_ops *ops;