[PATCH v8 14/14] iommu/arm-smmu-v3: Set MEV bit in nested STE for DoS mitigations

Nicolin Chen
2025-02-25T17:26:42Z

There is a DoS concern on the shared hardware event queue among devices
passed through to VMs, that too many translation failures that belong to
VMs could overflow the shared hardware event queue if those VMs or their
VMMs don't handle/recover the devices properly.

The MEV bit in the STE allows to configure the SMMU HW to merge similar
event records, though there is no guarantee. Set it in a nested STE for
DoS mitigations.

In the future, we might want to enable the MEV for non-nested cases too
such as domain->type == IOMMU_DOMAIN_UNMANAGED or even IOMMU_DOMAIN_DMA.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Pranjal Shrivastavat <praan@google.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h | 1 +
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 2 ++
drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 4 ++--
3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
index f3c5c49bf131..bc4f536f72ce 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h
@@ -266,6 +266,7 @@ static inline u32 arm_smmu_strtab_l2_idx(u32 sid)
#define STRTAB_STE_1_S1COR GENMASK_ULL(5, 4)
#define STRTAB_STE_1_S1CSH GENMASK_ULL(7, 6)

+#define STRTAB_STE_1_MEV (1UL << 19)
#define STRTAB_STE_1_S2FWB (1UL << 25)
#define STRTAB_STE_1_S1STALLD (1UL << 27)

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 649e3aa39a48..8e8ea3702ce5 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
@@ -43,6 +43,8 @@ static void arm_smmu_make_nested_cd_table_ste(
target->data[0] |= nested_domain->ste[0] &
~cpu_to_le64(STRTAB_STE_0_CFG);
target->data[1] |= nested_domain->ste[1];
+ /* Merge events for DoS mitigations on eventq */
+ target->data[1] |= cpu_to_le64(STRTAB_STE_1_MEV);
}

/*
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 22aa5c8d1e9d..3fcb1089a7c7 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -1052,7 +1052,7 @@ void arm_smmu_get_ste_used(const __le64 *ent, __le64 *used_bits)
cpu_to_le64(STRTAB_STE_1_S1DSS | STRTAB_STE_1_S1CIR |
STRTAB_STE_1_S1COR | STRTAB_STE_1_S1CSH |
STRTAB_STE_1_S1STALLD | STRTAB_STE_1_STRW |
- STRTAB_STE_1_EATS);
+ STRTAB_STE_1_EATS | STRTAB_STE_1_MEV);
used_bits[2] |= cpu_to_le64(STRTAB_STE_2_S2VMID);

/*
@@ -1068,7 +1068,7 @@ void arm_smmu_get_ste_used(const __le64 *ent, __le64 *used_bits)
if (cfg & BIT(1)) {
used_bits[1] |=
cpu_to_le64(STRTAB_STE_1_S2FWB | STRTAB_STE_1_EATS |
- STRTAB_STE_1_SHCFG);
+ STRTAB_STE_1_SHCFG | STRTAB_STE_1_MEV);
used_bits[2] |=
cpu_to_le64(STRTAB_STE_2_S2VMID | STRTAB_STE_2_VTCR |
STRTAB_STE_2_S2AA64 | STRTAB_STE_2_S2ENDI |
--
2.43.0