Hi Baolu,

On Tue, Jan 12, 2021 at 12:31:23PM +0800, Lu Baolu wrote:

Hi Jean,

On 1/8/21 10:52 PM, Jean-Philippe Brucker wrote:

quoted

Some devices manage I/O Page Faults (IOPF) themselves instead of relying
on PCIe PRI or Arm SMMU stall. Allow their drivers to enable SVA without
mandating IOMMU-managed IOPF. The other device drivers now need to first
enable IOMMU_DEV_FEAT_IOPF before enabling IOMMU_DEV_FEAT_SVA.

Signed-off-by: Jean-Philippe Brucker <redacted>
---
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: Zhangfei Gao <zhangfei.gao@linaro.org>
Cc: Zhou Wang <wangzhou1@hisilicon.com>
---
  include/linux/iommu.h | 20 +++++++++++++++++---
  1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 583c734b2e87..701b2eeb0dc5 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h

@@ -156,10 +156,24 @@ struct iommu_resv_region {
  	enum iommu_resv_type	type;
  };
-/* Per device IOMMU features */
+/**
+ * enum iommu_dev_features - Per device IOMMU features
+ * @IOMMU_DEV_FEAT_AUX: Auxiliary domain feature
+ * @IOMMU_DEV_FEAT_SVA: Shared Virtual Addresses
+ * @IOMMU_DEV_FEAT_IOPF: I/O Page Faults such as PRI or Stall. Generally using
+ *			 %IOMMU_DEV_FEAT_SVA requires %IOMMU_DEV_FEAT_IOPF, but
+ *			 some devices manage I/O Page Faults themselves instead
+ *			 of relying on the IOMMU. When supported, this feature
+ *			 must be enabled before and disabled after
+ *			 %IOMMU_DEV_FEAT_SVA.

Is this only for SVA? We may see more scenarios of using IOPF. For
example, when passing through devices to user level, the user's pages
could be managed dynamically instead of being allocated and pinned
statically.

Hm, isn't that precisely what SVA does?  I don't understand the
difference. That said FEAT_IOPF doesn't have to be only for SVA. It could
later be used as a prerequisite some another feature. For special cases
device drivers can always use the iommu_register_device_fault_handler()
API and handle faults themselves.

If @IOMMU_DEV_FEAT_IOPF is defined as generic iopf support, the current
vendor IOMMU driver support may not enough.

IOMMU_DEV_FEAT_IOPF on its own doesn't do anything useful, it's mainly a
way for device drivers to probe the IOMMU capability. Granted in patch
10 the SMMU driver registers the IOPF queue on enable() but that could be
done by FEAT_SVA enable() instead, if we ever repurpose FEAT_IOPF.

Thanks,
Jean