Re: [PATCH v1 1/8] iommu: Add iommu_group_replace_domain()

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

On 2022-02-14 14:56, Jason Gunthorpe via iommu wrote:
> On Mon, Feb 14, 2022 at 02:10:19PM +0000, Robin Murphy wrote:
>> On 2022-02-14 12:45, Jason Gunthorpe wrote:
>>> On Mon, Feb 14, 2022 at 12:09:36PM +0000, Robin Murphy wrote:
>>>> On 2022-01-06 02:20, Lu Baolu wrote:
>>>>> Expose an interface to replace the domain of an iommu group for frameworks
>>>>> like vfio which claims the ownership of the whole iommu group.
>>>>
>>>> But if the underlying point is the new expectation that
>>>> iommu_{attach,detach}_device() operate on the device's whole group where
>>>> relevant, why should we invent some special mechanism for VFIO to be
>>>> needlessly inconsistent?
>>>>
>>>> I said before that it's trivial for VFIO to resolve a suitable device if it
>>>> needs to; by now I've actually written the patch ;)
>>>>
>>>> https://gitlab.arm.com/linux-arm/linux-rm/-/commit/9f37d8c17c9b606abc96e1f1001c0b97c8b93ed5
>>>
>>> Er, how does locking work there? What keeps busdev from being
>>> concurrently unplugged?
>>
>> Same thing that prevents the bus pointer from suddenly becoming invalid in
>> the current code, I guess :)
> 
> Oooh, yes, that does look broken now too. :(
> 
>>> How can iommu_group_get() be safely called on
>>> this pointer?
>>
>> What matters is being able to call *other* device-based IOMMU API
>> interfaces in the long term.
> 
> Yes, this is what I mean, those are the ones that call
> iommu_group_get().
> 
>>> All of the above only works normally inside a probe/remove context
>>> where the driver core is blocking concurrent unplug and descruction.
>>>
>>> I think I said this last time you brought it up that lifetime was the
>>> challenge with this idea.
>>
>> Indeed, but it's a challenge that needs tackling, because the bus-based
>> interfaces need to go away. So either we figure it out now and let this
>> attach interface rework benefit immediately, or I spend three times as long
> 
> IMHO your path is easier if you let VFIO stay with the group interface
> and use something like:
> 
>     domain = iommu_group_alloc_domain(group)
> 
> Which is what VFIO is trying to accomplish. Since Lu removed the only
> other user of iommu_group_for_each_dev() it means we can de-export
> that interface.
> 
> This works better because the iommu code can hold the internal group
> while it finds the bus/device and then invokes the driver op. We don't
> have a lifetime problem anymore under that lock.

That's certainly one of the cleaner possibilities - per the theme of 
this thread I'm not hugely keen on proliferating special VFIO-specific 
versions of IOMMU APIs, but trying to take the dev->mutex might be a bit 
heavy-handed and risky, and getting at the vfio_group->device_lock a bit 
fiddly, so if I can't come up with anything nicer or more general it 
might be a fair compromise.

> The remaining VFIO use of bus for iommu_capable() is better done
> against the domain or the group object, as appropriate.

Indeed, although half the implementations of .capable are nonsense 
already, so I'm treating that one as a secondary priority for the moment 
(with an aim to come back afterwards and just try to kill it off as far 
as possible). RDMA and VFIO shouldn't be a serious concern for the kind 
of systems with heterogeneous IOMMUs at this point.

> In the bigger picture, VFIO should stop doing
> 'iommu_group_alloc_domain' by moving the domain alloc to
> VFIO_GROUP_GET_DEVICE_FD where we have a struct device to use.
> 
> We've already been experimenting with this for iommufd and the subtle
> difference in the uapi doesn't seem relevant.
> 
>> solving it on my own and end up deleting
>> iommu_group_replace_domain() in about 6 months' time anyway.
> 
> I expect this API to remain until we figure out a solution to the PPC
> problem, and come up with an alternative way to change the attached
> domain on the fly.

I though PPC wasn't using the IOMMU API at all... or is that the problem?

Thanks,
Robin.
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