Re: [PATCH 3/5] iommu/arm-smmu-v3: add IOMMU_CAP_BYPASS to the ARM SMMUv3 driver

[Anup Patel]

On Wed, Jul 19, 2017 at 4:30 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> On 19/07/17 10:33, Anup Patel wrote:
>> 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 <anup.patel@xxxxxxxxxxxx>
>> ---
>>  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.

Regards,
Anup