[RFC 1/7] ioasid: reserve special PASID for in-kernel DMA

[Jacob Pan]

IOASIDs associated with user processes are subject to resource
restrictions. However, in-kernel usages can target only one global
kernel virtual address mapping. Reserve a special IOASID for the devices
that perform DMA requests with PASID. This global PASID is excluded from
the IOASID allocator.

Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c | 2 +-
 drivers/iommu/ioasid.c                          | 2 ++
 drivers/iommu/iommu-sva-lib.c                   | 1 +
 include/linux/ioasid.h                          | 4 ++++
 4 files changed, 8 insertions(+), 1 deletion(-)

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 bb251cab61f3..c5fb89bd6229 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
@@ -329,7 +329,7 @@ __arm_smmu_sva_bind(struct device *dev, struct mm_struct *mm)
 		return ERR_PTR(-ENOMEM);
 
 	/* Allocate a PASID for this mm if necessary */
-	ret = iommu_sva_alloc_pasid(mm, 1, (1U << master->ssid_bits) - 1);
+	ret = iommu_sva_alloc_pasid(mm, IOASID_ALLOC_START, (1U << master->ssid_bits) - 1);
 	if (ret)
 		goto err_free_bond;
 
diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
index 50ee27bbd04e..89c6132bf1ec 100644
--- a/drivers/iommu/ioasid.c
+++ b/drivers/iommu/ioasid.c
@@ -317,6 +317,8 @@ ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
 	data->private = private;
 	refcount_set(&data->refs, 1);
 
+	if (min < IOASID_ALLOC_BASE)
+		min = IOASID_ALLOC_BASE;
 	/*
 	 * Custom allocator needs allocator data to perform platform specific
 	 * operations.
diff --git a/drivers/iommu/iommu-sva-lib.c b/drivers/iommu/iommu-sva-lib.c
index bd41405d34e9..4f56843517e5 100644
--- a/drivers/iommu/iommu-sva-lib.c
+++ b/drivers/iommu/iommu-sva-lib.c
@@ -28,6 +28,7 @@ int iommu_sva_alloc_pasid(struct mm_struct *mm, ioasid_t min, ioasid_t max)
 	int ret = 0;
 	ioasid_t pasid;
 
+	/* TODO: no need to check!! reserved range is checked in ioasid_alloc() */
 	if (min == INVALID_IOASID || max == INVALID_IOASID ||
 	    min == 0 || max < min)
 		return -EINVAL;
diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
index e9dacd4b9f6b..4d435cbd48b8 100644
--- a/include/linux/ioasid.h
+++ b/include/linux/ioasid.h
@@ -6,6 +6,10 @@
 #include <linux/errno.h>
 
 #define INVALID_IOASID ((ioasid_t)-1)
+#define IOASID_DMA_NO_PASID	0 /* For DMA request w/o PASID */
+#define IOASID_DMA_PASID	1 /* For in-kernel DMA w/ PASID */
+#define IOASID_ALLOC_BASE	2 /* Start of the allocation */
+
 typedef unsigned int ioasid_t;
 typedef ioasid_t (*ioasid_alloc_fn_t)(ioasid_t min, ioasid_t max, void *data);
 typedef void (*ioasid_free_fn_t)(ioasid_t ioasid, void *data);
-- 
2.25.1