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 4:55 PM, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Wed, Jul 19, 2017 at 04:53:04PM +0530, Anup Patel wrote:
>> 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.
>
> 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.

Regards,
Anup