[PATCH v3 3/4] iommu: Redefine IOMMU_CAP_CACHE_COHERENCY as the cap flag for IOMMU_CACHE

[Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>]

While the comment was correct that this flag was intended to convey the
block no-snoop support in the IOMMU, it has become widely implemented and
used to mean the IOMMU supports IOMMU_CACHE as a map flag. Only the Intel
driver was different.

Now that the Intel driver is using enforce_cache_coherency() update the
comment to make it clear that IOMMU_CAP_CACHE_COHERENCY is only about
IOMMU_CACHE.  Fix the Intel driver to return true since IOMMU_CACHE always
works.

The two places that test this flag, usnic and vdpa, are both assigning
userspace pages to a driver controlled iommu_domain and require
IOMMU_CACHE behavior as they offer no way for userspace to synchronize
caches.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/iommu/intel/iommu.c | 2 +-
 include/linux/iommu.h       | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
index 2332060e059c3d..4aebf8fa6d119a 100644
--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -4556,7 +4556,7 @@ static bool intel_iommu_enforce_cache_coherency(struct iommu_domain *domain)
 static bool intel_iommu_capable(enum iommu_cap cap)
 {
 	if (cap == IOMMU_CAP_CACHE_COHERENCY)
-		return domain_update_iommu_snooping(NULL);
+		return true;
 	if (cap == IOMMU_CAP_INTR_REMAP)
 		return irq_remapping_enabled == 1;
 
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index fe4f24c469c373..fd58f7adc52796 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -103,8 +103,7 @@ static inline bool iommu_is_dma_domain(struct iommu_domain *domain)
 }
 
 enum iommu_cap {
-	IOMMU_CAP_CACHE_COHERENCY,	/* IOMMU can enforce cache coherent DMA
-					   transactions */
+	IOMMU_CAP_CACHE_COHERENCY,	/* IOMMU_CACHE is supported */
 	IOMMU_CAP_INTR_REMAP,		/* IOMMU supports interrupt isolation */
 	IOMMU_CAP_NOEXEC,		/* IOMMU_NOEXEC flag */
 };
-- 
2.35.1

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