iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

PASIDs are process-wide. It was attempted to use refcounted PASIDs to
free them when the last thread drops the refcount. This turned out to
be complex and error prone. Given the fact that the PASID space is 20
bits, which allows up to 1M processes to have a PASID associated
concurrently, PASID resource exhaustion is not a realistic concern.

Therefore, it was decided to simplify the approach and stick with lazy
on demand PASID allocation, but drop the eager free approach and make an
allocated PASID's lifetime bound to the lifetime of the process.

Get rid of the refcounting mechanisms and replace/rename the interfaces
to reflect this new approach.

  [ bp: Massage commit message. ]

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20220207230254.3342514-6-fenghua.yu@intel.com

1 files changed, 12 insertions, 27 deletions

diff --git a/drivers/iommu/iommu-sva-lib.c b/drivers/iommu/iommu-sva-lib.c
index bd41405d34e9..106506143896 100644
--- a/drivers/iommu/iommu-sva-lib.c
+++ b/drivers/iommu/iommu-sva-lib.c
@@ -18,8 +18,7 @@ static DECLARE_IOASID_SET(iommu_sva_pasid);
  *
  * Try to allocate a PASID for this mm, or take a reference to the existing one
  * provided it fits within the [@min, @max] range. On success the PASID is
- * available in mm->pasid, and must be released with iommu_sva_free_pasid().
- * @min must be greater than 0, because 0 indicates an unused mm->pasid.
+ * available in mm->pasid and will be available for the lifetime of the mm.
  *
  * Returns 0 on success and < 0 on error.
  */
@@ -33,38 +32,24 @@ int iommu_sva_alloc_pasid(struct mm_struct *mm, ioasid_t min, ioasid_t max)
 		return -EINVAL;
 
 	mutex_lock(&iommu_sva_lock);
-	if (mm->pasid) {
-		if (mm->pasid >= min && mm->pasid <= max)
-			ioasid_get(mm->pasid);
-		else
+	/* Is a PASID already associated with this mm? */
+	if (pasid_valid(mm->pasid)) {
+		if (mm->pasid < min || mm->pasid >= max)
 			ret = -EOVERFLOW;
-	} else {
-		pasid = ioasid_alloc(&iommu_sva_pasid, min, max, mm);
-		if (pasid == INVALID_IOASID)
-			ret = -ENOMEM;
-		else
-			mm->pasid = pasid;
+		goto out;
 	}
+
+	pasid = ioasid_alloc(&iommu_sva_pasid, min, max, mm);
+	if (!pasid_valid(pasid))
+		ret = -ENOMEM;
+	else
+		mm_pasid_set(mm, pasid);
+out:
 	mutex_unlock(&iommu_sva_lock);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(iommu_sva_alloc_pasid);
 
-/**
- * iommu_sva_free_pasid - Release the mm's PASID
- * @mm: the mm
- *
- * Drop one reference to a PASID allocated with iommu_sva_alloc_pasid()
- */
-void iommu_sva_free_pasid(struct mm_struct *mm)
-{
-	mutex_lock(&iommu_sva_lock);
-	if (ioasid_put(mm->pasid))
-		mm->pasid = 0;
-	mutex_unlock(&iommu_sva_lock);
-}
-EXPORT_SYMBOL_GPL(iommu_sva_free_pasid);
-
 /* ioasid_find getter() requires a void * argument */
 static bool __mmget_not_zero(void *mm)
 {