Re: [PATCH 3/6] vfio: remove the unused mdev iommu hook

Robin Murphy
2021-05-19T15:23:35Z

On 2021-05-17 16:35, Joerg Roedel wrote:
> On Mon, May 17, 2021 at 10:35:00AM -0300, Jason Gunthorpe wrote:
>> Well, I'm sorry, but there is a huge other thread talking about the
>> IOASID design in great detail and why this is all needed. Jumping into
>> this thread without context and basically rejecting all the
>> conclusions that were reached over the last several weeks is really
>> not helpful - especially since your objection is not technical.
>>
>> I think you should wait for Intel to put together the /dev/ioasid uAPI
>> proposal and the example use cases it should address then you can give
>> feedback there, with proper context.
>
> Yes, I think the next step is that someone who read the whole thread
> writes up the conclusions and a rough /dev/ioasid API proposal, also
> mentioning the use-cases it addresses. Based on that we can discuss the
> implications this needs to have for IOMMU-API and code.
>
> From the use-cases I know the mdev concept is just fine. But if there is
> a more generic one we can talk about it.

Just to add another voice here, I have some colleagues working on
drivers where they want to use SMMU Substream IDs for a single hardware
block to operate on multiple iommu_domains managed entirely within the
kernel. Using an mdev-like approach with aux domains is pretty much the
ideal fit for this use-case, while all the IOASID discussion appears
centred on SVA and userspace interfaces, and as such barely relevant if
at all.

I seem to recall a non-trivial amount of effort going into the aux
domain design too, so the promise of replacing it with "a big TBD" just
because vfio-mdev turned out to be awful hardly fills me with enthusiasm
either :/

Robin.