[Nouveau] CUDA fixed VA allocations and sparse mappings

[Andrew Chew]

On Wed, Jul 08, 2015 at 10:18:36AM +1000, Ben Skeggs wrote:
> > There's some minimal state that needs to be mapped into GPU address space.
> > One thing that comes to mind are pushbuffers, which are needed to submit
> > stuff to any engine.
> I guess you can probably use the start of the kernel's address space
> carveout for these kind of mappings actually?  It's not like userspace
> can ever have virtual addresses there?

Yeah.  I'm looking into it further, but to answer your original question,
I believe there is essentially an address range that nouveau would know
about, which it uses for fixed address allocations (I'm referring to how
the nvgpu driver does things...we may or may not come up with something
different for nouveau).

Although it's dangerous, AFAIK the allocator in nouveau starts allocating
addresses at page 1, and as you suggested, one wouldn't ever get a CPU
address that low.  But having a set of addresses reserved would be much
better of course.