[Nouveau] CUDA fixed VA allocations and sparse mappings

[Ben Skeggs]

On 8 July 2015 at 10:47, Andrew Chew <achew at nvidia.com> wrote:
> 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...
After a very quick look, it looks like the kernel defines a
PAGE_OFFSET macro which is the start of kernel virtual address space.

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