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

[Robin Murphy <robin.murphy@arm.com>]

On 20/07/17 10:10, Will Deacon wrote:
> On Thu, Jul 20, 2017 at 09:32:00AM +0530, Anup Patel wrote:
>> On Wed, Jul 19, 2017 at 5:23 PM, Will Deacon <will.deacon@arm.com> wrote:
>>> There are two things here:
>>>
>>>   1. iommu_present() is pretty useless, because it applies to a "bus" which
>>>      doesn't actually tell you what you need to know for things like the
>>>      platform_bus, where some masters might be upstream of an SMMU and
>>>      others might not be.
>>
>> I agree with you. The iommu_present() check in vfio_iommu_group_get()
>> is not much useful. We only reach line which checks iommu_present()
>> when iommu_group_get() returns NULL for given "struct device *". If there
>> is no IOMMU group for a "struct device *" then it means there is no IOMMU
>> HW doing translations for such device.
>>
>> If we drop the iommu_present() check (due to above reasons) in
>> vfio_iommu_group_get() then we don't require the IOMMU_CAP_BYPASS
>> and we can happily drop PATCH1, PATCH2, and PATCH3.
>>
>> I will remove the iommu_present() check in vfio_iommu_group_get()
>> because it is only comes into actions when VFIO_NOIOMMU is
>> enabled. This will also help us drop PATCH1-to-PATCH3.
> 
> I don't think that's the right answer. Whilst iommu_present has obvious
> shortcomings, its intention is clear: it should tell you whether a given
> *device* is upstream of an IOMMU. So the right fix is to make this
> per-device, instead of per-bus. Removing it altogether is worse than leaving
> it like it is.

Not really - if there is an IOMMU up and running to the point of setting
bus ops, every device it cares about can be expected to have a group
already (there are only a couple of drivers left that don't use groups,
and they're hardly relevant to VFIO). Thus iommu_group_get() already is
the de-facto per-device IOMMU check.

And having looked into it, I'm now spinning a couple of patches to
finish off making groups truly mandatory so that that can be less
de-facto ;)

Robin.

>>>   2. If a master *is* upstream of an IOMMU and you want to use no-IOMMU,
>>>      then the VFIO no-IOMMU code needs to be extended so that it creates
>>>      an IDENTITY domain on that IOMMU.
>>
>> The VFIO no-IOMMU mode is equivalent to Linux UIO hence having
>> IDENTITY domain for VFIO no-IOMMU is not appropriate here.
> 
> Can you elaborate on this please? I don't understand the argument you're
> making. It's like saying "I don't like eggs, therefore I don't drive a
> car".
> 
> Will
>