On 23/04/2022 20.30, Jens Axboe wrote:
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.
Sorry, I caused a lot of confusion by bundling immediate copy and a DMA
engine interface. For sure the immediate copy should be a direct
implementation like you posted!
User-to-user copies are another matter. I feel like that should be a
stand-alone driver, and that io_uring should be an io_uring-y way to
access it. Just like io_uring isn't an NVMe driver.
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.
Of course, but bikeshedding from the sidelines never hurt anyone.
For outside of io_uring, you're looking at a sync
interface, which I think already exists for this (ioctls?).
Yes, it would be a asynchronous interface. I don't know if one exists,
but I can't claim to have kept track.
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.
For sure, I'm just noting that the DMA engine has many different uses
and so deserves an interface that is untied to io_uring.
