[Nouveau] CUDA fixed VA allocations and sparse mappings

[Andrew Chew]

Hello,

I am currently looking into ways to support fixed virtual address allocations
and sparse mappings in nouveau, as a step towards supporting CUDA.

CUDA requires that the GPU virtual address for a given buffer match the
CPU virtual address.  Therefore, when mapping a CUDA buffer, we have to have
a way of specifying a particular virtual address to map to (we would ask that
the CPU virtual address be used).  Currently, as I understand it, the allocator
implemented in nvkm/core/mm.c, used to provision virtual addresses, doesn't
allow for this (but it's very easy to modify the allocator slightly to allow
for this, which I have done locally in my experiments).

In addition, the CUDA use case typically involves allocating a big chunk of
address space ahead of time as a way to reserve that chunk for future CUDA
use.  It then maps individual buffers into that address space as needed.
Currently, the virtual address allocation is done during buffer mapping, so
in order to support these sparse mappings, it seems to me that the virtual
address allocation and buffer mapping need to be decoupled into separate
operations.

My current strawman proposal for supporting this is to introduce two new ioctls
DRM_IOCTL_NOUVEAU_AS_ALLOC and DRM_IOCTL_NOUVEAU_AS_FREE, that look roughly
like this:

#define NOUVEAU_AS_ALLOC_FLAGS_FIXED_OFFSET 0x1
struct drm_nouveau_as_alloc {
        uint64_t pages;     /* in, pages */
        uint32_t page_size; /* in, bytes */
        uint32_t flags;     /* in */
        uint64_t offset;    /* in/out, byte address */
};

struct drm_nouveau_as_free {
        uint64_t offset;    /* in, byte address */
};

These ioctls just call into the allocator to allocate a range of addresses,
resulting in a struct nvkm_vma that tracks that allocation (or releases the
struct nvkm_vma back into the virtual address pool in the case of the free
ioctl).  If NOUVEAU_AS_ALLOC_FLAGS_FIXED_OFFSET is set, offset specifies the
requested virtual address.  Otherwise, an arbitrary address will be
allocated.

In addition to this, a way to map/unmap buffers is needed.  Ordinarily, one
would just use DRM_IOCTL_PRIME_FD_TO_HANDLE to import and map a dmabuf into
gem.  However, this ioctl will try to grab the virtual address range for this
buffer, which will fail in the CUDA case since the virtual address range
has been reserved ahead of time.  So we perhaps introduce a set of ioctls
to map/unmap buffers on top of an already existing virtual address allocation.

Please, feedback and questions are very much appreciated.