This is certainly not the first time this has been brought up, but I'd like to try and
get some consensus on the best way to move this forward. Allowing devices to talk
directly improves performance and reduces latency by avoiding the use of staging buffers
in system memory. Also in cases where both devices are behind a switch, it avoids the CPU
entirely. Most current APIs (DirectGMA, PeerDirect, CUDA, HSA) that deal with this are
pointer based. Ideally we'd be able to take a CPU virtual address and be able to get
to a physical address taking into account IOMMUs, etc. Having struct pages for the memory
would allow it to work more generally and wouldn't require as much explicit support in
drivers that wanted to use it.
Some use cases:
1. Storage devices streaming directly to GPU device memory
2. GPU device memory to GPU device memory streaming
3. DVB/V4L/SDI devices streaming directly to GPU device memory
4. DVB/V4L/SDI devices streaming directly to storage devices
Here is a relatively simple example of how this could work for testing. This is obviously
not a complete solution.
- Device memory will be registered with Linux memory sub-system by created corresponding
struct page structures for device memory
- get_user_pages_fast() will return corresponding struct pages when CPU address points to
the device memory
- put_page() will deal with struct pages for device memory
Previously proposed solutions and related proposals:
DMA-API/PCI map_peer_resource support for peer-to-peer
Pros: Low impact, already largely reviewed.
Cons: requires explicit support in all drivers that want to support it, doesn't handle
S/G in device memory.
2. ZONE_DEVICE IO
Direct I/O and DMA for persistent memory (https://lwn.net/Articles/672457/
Add support for ZONE_DEVICE IO memory with struct pages.
Pro: Doesn't waste system memory for ZONE metadata
Cons: CPU access to ZONE metadata slow, may be lost, corrupted on device reset.
RDMA subsystem DMA-BUF support (http://www.spinics.net/lists/linux-rdma/msg38748.html
Pros: uses existing dma-buf interface
Cons: dma-buf is handle based, requires explicit dma-buf support in drivers.
iopmem : A block device for PCIe memory (https://lwn.net/Articles/703895/
Heterogeneous Memory Management
6. Some new mmap-like interface that takes a userptr and a length and returns a dma-buf