memory access op ideas

[Avi Kivity <avi@scylladb.com>]

Unfortunately, only ideas, no patches. But at least the first seems very 
easy.


- IORING_OP_MEMCPY_IMMEDIATE - copy some payload included in the op 
itself (1-8 bytes) to a user memory location specified by the op.


Linked to another op, this can generate an in-memory notification useful 
for busy-waiters or the UMWAIT instruction


This would be useful for Seastar, which looks at a timer-managed memory 
location to check when to break computation loops.


- IORING_OP_MEMCPY - asynchronously copy memory


Some CPUs include a DMA engine, and io_uring is a perfect interface to 
exercise it. It may be difficult to find space for two iovecs though.