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

[Will Deacon <will.deacon@arm.com>]

On Wed, Jul 19, 2017 at 05:01:11PM +0530, Anup Patel wrote:
> On Wed, Jul 19, 2017 at 4:55 PM, Will Deacon <will.deacon@arm.com> 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@arm.com> 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@broadcom.com>
> >> >> ---
> >> >>  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