[PATCH RFC 4/4] iommu/arm-smmu-v3: Wire up shared-SID STE ordering and feature gating
From: Peng Fan (OSS)
Date: Wed Sep 16 2026 - 11:45:48 EST
From: Peng Fan <peng.fan@xxxxxxx>
For shared SIDs, only the first master to attach writes the STE
(tracked by ste_installed under streams_mutex); subsequent masters
skip the write. On teardown, only the last master writes the ABORT
STE (ref_count == 1). This is handled by arm_smmu_skip_shared_ste().
Fault events on shared SIDs cannot be attributed to a specific master,
so arm_smmu_find_master() returns NULL when ref_count > 1.
Disable SVA, IOPF/stall, and vSMMU nesting for shared-SID masters
since all three require unambiguous SID-to-device mapping.
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
---
.../iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 4 +-
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c | 7 +++
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 59 ++++++++++++++++++++--
3 files changed, 65 insertions(+), 5 deletions(-)
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
index ab1078a97d801..258ca42917f07 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c
@@ -308,8 +308,10 @@ static int arm_vsmmu_vdevice_init(struct iommufd_vdevice *vdev)
/*
* arm_vsmmu_vsid_to_sid() maps a vSID to master->streams[0] alone, so
* more streams would leave the rest stale and none reads out of bounds.
+ * Shared SIDs are also unsupported for vSMMU since the STE is shared
+ * between multiple masters.
*/
- if (master->num_streams != 1)
+ if (master->num_streams != 1 || master->shared_sid)
return -EOPNOTSUPP;
return 0;
}
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
index 0a429c64fbf3e..43f8cc81bfdd1 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
@@ -271,6 +271,13 @@ static int arm_smmu_sva_set_dev_pasid(struct iommu_domain *domain,
if (!(master->smmu->features & ARM_SMMU_FEAT_SVA))
return -EOPNOTSUPP;
+ /*
+ * SVA requires stall-based fault handling which cannot be supported
+ * when multiple devices share a SID (fault routing is ambiguous).
+ */
+ if (master->shared_sid)
+ return -EOPNOTSUPP;
+
/* Prevent arm_smmu_mm_release from being called while we are attaching */
if (!mmget_not_zero(domain->mm))
return -EINVAL;
diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index bb3ee25d10d6e..13968191b1452 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -2034,6 +2034,13 @@ arm_smmu_find_master(struct arm_smmu_device *smmu, u32 sid)
stream = xa_load(&smmu->streams, sid);
if (!stream)
return NULL;
+ /*
+ * For shared SIDs (ref_count > 1) we cannot determine which master
+ * triggered the fault, so return NULL to let the caller handle it
+ * as an unresolvable event.
+ */
+ if (stream->ref_count > 1)
+ return NULL;
return stream->master;
}
@@ -2944,11 +2951,43 @@ arm_smmu_get_step_for_sid(struct arm_smmu_device *smmu, u32 sid)
}
}
+/*
+ * For shared SIDs, check whether this master should skip the STE write.
+ *
+ * Setup path: only the first master writes the STE; subsequent masters
+ * skip because ste_installed is already true.
+ *
+ * Teardown path (ABORT): skip while other masters still share the SID
+ * (ref_count > 1). Only the last remaining master writes the ABORT STE.
+ *
+ * Returns true if the STE write should be skipped for this SID.
+ */
+static bool arm_smmu_skip_shared_ste(struct arm_smmu_device *smmu,
+ u32 sid, bool is_abort)
+{
+ struct arm_smmu_stream *stream;
+ bool skip = false;
+
+ mutex_lock(&smmu->streams_mutex);
+ stream = xa_load(&smmu->streams, sid);
+ if (stream) {
+ if (is_abort)
+ skip = stream->ref_count > 1;
+ else if (stream->ste_installed)
+ skip = true;
+ else
+ stream->ste_installed = true;
+ }
+ mutex_unlock(&smmu->streams_mutex);
+ return skip;
+}
+
void arm_smmu_install_ste_for_dev(struct arm_smmu_master *master,
const struct arm_smmu_ste *target)
{
int i, j;
struct arm_smmu_device *smmu = master->smmu;
+ bool is_abort;
master->cd_table.in_ste =
FIELD_GET(STRTAB_STE_0_CFG, le64_to_cpu(target->data[0])) ==
@@ -2957,10 +2996,12 @@ void arm_smmu_install_ste_for_dev(struct arm_smmu_master *master,
FIELD_GET(STRTAB_STE_1_EATS, le64_to_cpu(target->data[1])) ==
STRTAB_STE_1_EATS_TRANS;
+ is_abort = (FIELD_GET(STRTAB_STE_0_CFG, le64_to_cpu(target->data[0])) ==
+ STRTAB_STE_0_CFG_ABORT);
+
for (i = 0; i < master->num_streams; ++i) {
u32 sid = master->streams[i].id;
- struct arm_smmu_ste *step =
- arm_smmu_get_step_for_sid(smmu, sid);
+ struct arm_smmu_ste *step;
/* Bridged PCI devices may end up with duplicated IDs */
for (j = 0; j < i; j++)
@@ -2969,6 +3010,11 @@ void arm_smmu_install_ste_for_dev(struct arm_smmu_master *master,
if (j < i)
continue;
+ if (master->shared_sid &&
+ arm_smmu_skip_shared_ste(smmu, sid, is_abort))
+ continue;
+
+ step = arm_smmu_get_step_for_sid(smmu, sid);
arm_smmu_write_ste(master, sid, step, target);
}
}
@@ -3116,8 +3162,13 @@ static int arm_smmu_enable_iopf(struct arm_smmu_master *master,
if (!master->stall_enabled)
return 0;
- /* We're not keeping track of SIDs in fault events */
- if (master->num_streams != 1)
+ /*
+ * We're not keeping track of SIDs in fault events, and stall/PRI
+ * cannot be supported when multiple devices share a SID because page
+ * fault responses are routed by RID/SID and we cannot distinguish
+ * which device triggered the fault.
+ */
+ if (master->num_streams != 1 || master->shared_sid)
return -EOPNOTSUPP;
if (master->iopf_refcount) {
--
2.34.1