io_uring: add BPF-driven requests

Pretty much an RFC for executing BPF requests in io_uring to start a
discussion, so any ideas and wish lists are welcom. Some technical
but not much important for the discussion detais are omitted in the
patchset, i.e. good cancellation system, optimisations and some BPF
specific parts.

Some of major missing points:
1. We need to pass an CQE/result of a previous linked request to BPF.

2. Whether to keep to a new IORING_OP_BPF opcode, or do it in a common
path for each request, e.g. on CQE completion. The former looks saner,
but is not nicely aligned with (1.).

3. Allow BPF programs to generate CQEs not binded to a request. A
problem can be with supporting overflowed CQEs, it's either to
always kmalloc()'ing storage for them instead of using req's memory
or piling up on top. Eventually we will need it anyway to be able
to post several CQEs for a single requests.

Pavel Begunkov (4):
  bpf: add IOURING program type
  io_uring: implement bpf prog registration
  io_uring: add IORING_OP_BPF
  io_uring: enable BPF to submit SQEs

 fs/io_uring.c                 | 259 +++++++++++++++++++++++++++++++++-
 include/linux/bpf_types.h     |   2 +
 include/uapi/linux/bpf.h      |   2 +
 include/uapi/linux/io_uring.h |   3 +
 kernel/bpf/syscall.c          |   1 +
 kernel/bpf/verifier.c         |   3 +
 6 files changed, 264 insertions(+), 6 deletions(-)

-- 
2.24.0