On 9/4/19 8:58 AM, Christoph Hellwig wrote:

On Tue, Sep 03, 2019 at 10:46:18PM +0200, Thomas Hellström (VMware) wrote:

...

What I mean with "from an engineering perspective" is that drivers would end up with a non-trivial amount of code supporting purely academic cases: Setups where software rendering would be faster than gpu accelerated, and setups on platforms where the driver would never run anyway because the device would never be supported on that platform...

And actually work on cases you previously called academic and which now matter to you because your employer has a suddent interest in SEV. Academic really is in the eye of the beholder (and of those who pay the bills).

But in this particular case we *do* adhere to the dma api, at least as far as we can. But we're missing functionality.

...

That is not really true. The dma API can't handle faulting of coherent pages which is what this series is really all about supporting also with SEV active. To handle the case where we move graphics buffers or send them to swap space while user-space have them mapped.

And the only thing we need to support the fault handler is to add an offset to the dma_mmap_* APIs. Which I had planned to do for Christian (one of the few grapics developers who actually tries to play well with the rest of the kernel instead of piling hacks over hacks like many others) anyway, but which hasn't happened yet.

That sounds great. Is there anything I can do to help out? I thought this was more or less a dead end since the current dma_mmap_ API requires the mmap_sem to be held in write mode (modifying the vma->vm_flags) whereas fault() only offers read mode. But that would definitely work.

...

Still, I need a way forward and my questions weren't really answered by this.

This is pretty demanding. If you "need" a way forward just work with all the relevant people instead of piling ob local hacks.

But I think that was what I was trying to initiate. The question was

"If it's the latter, then I would like to reiterate that it would be better that we work to come up with a long term plan to add what's missing to the DMA api to help graphics drivers use coherent memory?"

And since you NAK'd the original patches, I was sort of hoping for a point in the right direction.

Thanks,

Thomas