Re: memory access op ideas

[Jens Axboe <axboe@kernel.dk>]

On 4/23/22 10:23 AM, Avi Kivity wrote:
> Perhaps the interface should be kept separate from io_uring. e.g. use
> a pidfd to represent the address space, and then issue
> IORING_OP_PREADV/IORING_OP_PWRITEV to initiate dma. Then one can copy
> across process boundaries.

Then you just made it a ton less efficient, particularly if you used the
vectored read/write. For this to make sense, I think it has to be a
separate op. At least that's the only implementation I'd be willing to
entertain for the immediate copy.

> A different angle is to use expose the dma device as a separate fd.
> This can be useful as dma engine can often do other operations, like
> xor or crc or encryption or compression. In any case I'd argue for the
> interface to be useful outside io_uring, although that considerably
> increases the scope. I also don't have a direct use case for it,
> though I'm sure others will.

I'd say that whoever does it get to at least dictate the initial
implementation. For outside of io_uring, you're looking at a sync
interface, which I think already exists for this (ioctls?).

> The kernel itself should find the DMA engine useful for things like
> memory compaction.

That's a very different use case though and just deals with wiring it up
internally.

Let's try and keep the scope here reasonable, imho nothing good comes
out of attempting to do all the things at once.

-- 
Jens Axboe