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

[Anup Patel <anup.patel@broadcom.com>]

On Wed, Jul 19, 2017 at 5:03 PM, Will Deacon <will.deacon@arm.com> wrote:
> 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.

Currently, the iommu_present() check in vfio_iommu_group_get()
is preventing us allow no-IOMMU mode for certain devices.

Is there a better replacement of iommu_present() check in
vfio_iommu_group_get()?

Regards,
Anup