[PATCH v2 4/4] vfio/pci: Allow MMIO regions to be exported through dma-buf

[Robin Murphy]

On 2022-09-07 16:23, Jason Gunthorpe wrote:
> On Wed, Sep 07, 2022 at 07:29:58AM -0700, Christoph Hellwig wrote:
>> On Wed, Sep 07, 2022 at 09:33:11AM -0300, Jason Gunthorpe wrote:
>>> Yes, you said that, and I said that when the AMD driver first merged
>>> it - but it went in anyhow and now people are using it in a bunch of
>>> places.
>>
>> drm folks made up their own weird rules, if they internally stick
>> to it they have to listen to it given that they ignore review comments,
>> but it violates the scatterlist API and has not business anywhere
>> else in the kernel.  And yes, there probably is a reason or two why
>> the drm code is unusually error prone.
> 
> That may be, but it is creating problems if DRM gets to do X crazy
> thing and nobody else can..
> 
> So, we have two issues here
> 
>   1) DMABUF abuses the scatter list, but this is very constrainted we have
>      this weird special "DMABUF scatterlist" that is only touched by DMABUF
>      importers. The imports signal that they understand the format with
>      a flag. This is ugly and would be nice to clean to a dma mapped
>      address list of some sort.
> 
>      I spent alot of time a few years ago removing driver touches of
>      the SGL and preparing the RDMA stack to do this kind of change, at
>      least.
> 
>   2) DMABUF abuses dma_map_resource() for P2P and thus doesn't work in
>      certain special cases.

FWIW, dma_map_resource() *is* for P2P in general. The classic case of 
one device poking at another's registers that was the original 
motivation is a standalone DMA engine reading/writing a peripheral 
device's FIFO, so the very similar inter-device doorbell signal is 
absolutely in scope too; VRAM might be a slightly greyer area, but if 
it's still not page-backed kernel memory then I reckon that's fair game.

The only trouble is that it's not geared for *PCI* P2P when that may or 
may not happen entirely upstream of IOMMU translation.

Robin.