Re: memory access op ideas

[Jens Axboe <axboe@kernel.dk>]

On 4/24/22 8:56 AM, Avi Kivity wrote:
> 
> On 24/04/2022 16.30, Jens Axboe wrote:
>> On 4/24/22 7:04 AM, Avi Kivity wrote:
>>> 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.
>> Not sure I understand your logic here or the io_uring vs nvme driver
>> reference, to be honest. io_uring _is_ a standalone way to access it,
>> you can use it sync or async through that.
>>
>> If you're talking about a standalone op vs being useful from a command
>> itself, I do think both have merit and I can see good use cases for
>> both.
> 
> 
> I'm saying that if dma is exposed to userspace, it should have a
> regular synchronous interface (maybe open("/dev/dma"), maybe something
> else). io_uring adds asynchrony to everything, but it's not
> everything's driver.

Sure, my point is that if/when someone wants to add that, they should be
free to do so. It's not a fair requirement to put on someone doing the
initial work on wiring this up. It may not be something they would want
to use to begin with, and it's perfectly easy to run io_uring in sync
mode should you wish to do so. The hard part is making the
issue+complete separate actions, rolling a sync API on top of that would
be trivial.

> Anyway maybe we drifted off somewhere and this should be decided by
> pragmatic concerns (like whatever the author of the driver prefers).

Indeed!

-- 
Jens Axboe