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

[Robin Murphy]

On 24/07/17 18:16, Alex Williamson wrote:
> On Thu, 20 Jul 2017 12:17:12 +0100
> Robin Murphy <robin.murphy@xxxxxxx> wrote:
> 
>> 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@xxxxxxx> 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 ;)
> 
> No, look at vfio-noiommu and even vfio-mdev devices for devices which
> have an iommu group but there is no physical iommu supporting them.
> iommu_present() is how we can distinguish these groups and therefore
> not generate a segfault in trying to use the full IOMMU API on them.

OK, so that means that the combination of vfio-noiommu and vfio-platform
is simply unusable, because iommu_present(&platform_bus_type) can give
such dangerous false positives too.

Robin.