[Nouveau] CUDA fixed VA allocations and sparse mappings

[Ben Skeggs]

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?

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