[Nouveau] CUDA fixed VA allocations and sparse mappings

[Andrew Chew]

On Wed, Jul 08, 2015 at 10:37:34AM +1000, Ben Skeggs wrote:
> On 8 July 2015 at 10:31, Andrew Chew <achew at nvidia.com> wrote:
> > 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.
> I'm thinking more about the top of the address space.  As I understand
> it, the kernel already splits the CPU virtual address space into
> user/system areas (3GiB/1GiB for 32-bit IIUC), or something very
> similar to that.
> 
> Perhaps, if we can get at that information, we can use those same
> definitions for GPU address space?

Ah, I get what you're saying.  Sure, I think that might be okay.  Not sure
how we would get at that information, though, and it would be horrible to
just bake it in somewhere.  I'm looking into how nvgpu driver does it...
maybe they have good reasons to do it the way they do.  Sorry if I go
quiet for a little bit...