[PATCH v2 1/3] iommu: Add per-domain strictness and combine with the global default

From: Douglas Anderson
Date: Thu Jun 24 2021 - 13:18:32 EST

Strictness has the semantic of being a per-domain property. This is
why iommu_get_dma_strict() takes a "struct iommu_domain" as a
parameter. Let's add knowledge to the "struct iommu_domain" so we can
know whether we'd like each domain to be strict.

In this patch nothing sets the per-domain strictness, it just paves
the way for future patches to do so.

Prior to this patch we could only affect strictness at a global
level. We'll still honor the global strictness level if it has been
explicitly set and it's stricter than the one requested per-domain.

NOTE: it's even more obvious that iommu_set_dma_strict() and
iommu_get_dma_strict() are non-symmetric after this change. However,
they have always been asymmetric by design [0].

The function iommu_get_dma_strict() should now make it super obvious
where strictness comes from and who overides who. Though the function
changed a bunch to make the logic clearer, the only two new rules
should be:
* Devices can force strictness for themselves, overriding the cmdline
"iommu.strict=0" or a call to iommu_set_dma_strict(false)).
* Devices can request non-strictness for themselves, assuming there
was no cmdline "iommu.strict=1" or a call to
iommu_set_dma_strict(true).

[0] https://lore.kernel.org/r/a023af85-5060-0a3c-4648-b00f8b8c0430@xxxxxxx/

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---
This patch clearly will cause conflicts if John Garry's patches [1]
land before it. It shouldn't be too hard to rebase,
though. Essentially with John's patches it'll be impossible for what's
called `cmdline_dma_strict` in my patch to be "default". It'll
probably make sense to rearrange the logic/names a bit though just to
make things clearer.

[1] https://lore.kernel.org/r/1624016058-189713-1-git-send-email-john.garry@xxxxxxxxxx/

Changes in v2:
- No longer based on changes adding strictness to "struct device"
- Updated kernel-parameters docs.

.../admin-guide/kernel-parameters.txt | 5 ++-
drivers/iommu/iommu.c | 43 +++++++++++++++----
include/linux/iommu.h | 7 +++
3 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index cb89dbdedc46..7675fd79f9a9 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1995,9 +1995,12 @@
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
synchronously.
+ NOTE: if "iommu.strict" is not specified in the command
+ line then it's up to the system to try to determine the
+ proper strictness.

iommu.passthrough=
[ARM64, X86] Configure DMA to bypass the IOMMU by default.
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 808ab70d5df5..7943d2105b2f 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -29,7 +29,8 @@ 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 enum iommu_strictness cmdline_dma_strict __read_mostly;
+static enum iommu_strictness driver_dma_strict __read_mostly;
static u32 iommu_cmd_line __read_mostly;

struct iommu_group {
@@ -69,7 +70,6 @@ static const char * const iommu_group_resv_type_string[] = {
};

#define IOMMU_CMD_LINE_DMA_API BIT(0)
-#define IOMMU_CMD_LINE_STRICT BIT(1)

static int iommu_alloc_default_domain(struct iommu_group *group,
struct device *dev);
@@ -334,27 +334,52 @@ static int __init iommu_set_def_domain_type(char *str)
}
early_param("iommu.passthrough", iommu_set_def_domain_type);

+static inline enum iommu_strictness bool_to_strictness(bool strict)
+{
+ return strict ? IOMMU_STRICT : IOMMU_NOT_STRICT;
+}
+
static int __init iommu_dma_setup(char *str)
{
- int ret = kstrtobool(str, &iommu_dma_strict);
+ bool strict;
+ int ret = kstrtobool(str, &strict);

if (!ret)
- iommu_cmd_line |= IOMMU_CMD_LINE_STRICT;
+ cmdline_dma_strict = bool_to_strictness(strict);
return ret;
}
early_param("iommu.strict", iommu_dma_setup);

void iommu_set_dma_strict(bool strict)
{
- if (strict || !(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
- iommu_dma_strict = strict;
+ /*
+ * Valid transitions:
+ * - DEFAULT -> NON_STRICT
+ * - DEFAULT -> STRICT
+ * - NON_STRICT -> STRICT
+ *
+ * Everything else is ignored.
+ */
+ if (driver_dma_strict != IOMMU_STRICT)
+ driver_dma_strict = bool_to_strictness(strict);
}

bool iommu_get_dma_strict(struct iommu_domain *domain)
{
- /* only allow lazy flushing for DMA domains */
- if (domain->type == IOMMU_DOMAIN_DMA)
- return iommu_dma_strict;
+ /* Non-DMA domains or anyone forcing it to strict makes it strict */
+ if (domain->type != IOMMU_DOMAIN_DMA ||
+ cmdline_dma_strict == IOMMU_STRICT ||
+ driver_dma_strict == IOMMU_STRICT ||
+ domain->strictness == IOMMU_STRICT)
+ return true;
+
+ /* Anyone requesting non-strict (if no forces) makes it non-strict */
+ if (cmdline_dma_strict == IOMMU_NOT_STRICT ||
+ driver_dma_strict == IOMMU_NOT_STRICT ||
+ domain->strictness == IOMMU_NOT_STRICT)
+ return false;
+
+ /* Nobody said anything, so it's strict by default */
return true;
}
EXPORT_SYMBOL_GPL(iommu_get_dma_strict);
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 32d448050bf7..2e172059c931 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -79,8 +79,15 @@ struct iommu_domain_geometry {
#define IOMMU_DOMAIN_DMA (__IOMMU_DOMAIN_PAGING | \
__IOMMU_DOMAIN_DMA_API)

+enum iommu_strictness {
+ IOMMU_DEFAULT_STRICTNESS = 0, /* zero-init ends up at default */
+ IOMMU_NOT_STRICT,
+ IOMMU_STRICT,
+};
+
struct iommu_domain {
unsigned type;
+ enum iommu_strictness strictness;
const struct iommu_ops *ops;
unsigned long pgsize_bitmap; /* Bitmap of page sizes in use */
iommu_fault_handler_t handler;
--
2.32.0.93.g670b81a890-goog