On Wed, Jul 19, 2017 at 05:01:11PM +0530, Anup Patel wrote:

On Wed, Jul 19, 2017 at 4:55 PM, Will Deacon [off-list ref] wrote:

quoted

On Wed, Jul 19, 2017 at 04:53:04PM +0530, Anup Patel wrote:

quoted

On Wed, Jul 19, 2017 at 4:30 PM, Robin Murphy [off-list ref] wrote:

quoted

On 19/07/17 10:33, Anup Patel wrote:

quoted

The ARM SMMUv3 support bypassing transactions for which domain
is not configured. The patch adds corresponding IOMMU capability
to advertise this fact.

Signed-off-by: Anup Patel <redacted>
---
 drivers/iommu/arm-smmu-v3.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index 568c400..a6c7f66 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c

@@ -1423,6 +1423,8 @@ static bool arm_smmu_capable(enum iommu_cap cap)
              return true;
      case IOMMU_CAP_NOEXEC:
              return true;
+     case IOMMU_CAP_BYPASS:
+             return true;

And this is never true. If Linux knows a device masters through the
SMMU, it will always have a default domain of some sort (either identity
or DMA ops). If Linux doesn't know, then it won't have been able to
initialise the stream table for the relevant stream IDs, thus any
'bypass' DMA is going to raise C_BAD_STE. SMMUv3 can effectively only
bypass unknown stream IDs if disabled entirely.

What if we don't want to use IOMMU for certain device and
due to this we never provide "iommus" DT attribute in the
device DT node. Further, we want to access device without
"iommus" DT attribute from user-space using VFIO no-IOMMU.

Wait, you want to pass a device through to userspace but you don't want to
use the IOMMU? Why not?

If you describe the SMMU in firmware with only a partial topology
description, then you will run into problems with unknown masters trying to
perform DMA. That's the IOMMU doing its job!

We are keeping disable_bypass = false. In other words, we
are using bypass mode for unmatched streams. The real
reason is limited number of SMRs due to which we choose
not to provide "iommus" DT attribute for certain devices.

Understood, but that's not robust for SMMUv3 and we *really* shouldn't have
a user ABI that changes behaviour based on a cmdline option. VFIO should be
requesting its own identity mappings, if that's what you need.

Will