Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

From: Logan Gunthorpe
Date: Thu Jun 27 2019 - 12:10:00 EST

On 2019-06-27 12:32 a.m., Jason Gunthorpe wrote:
> On Wed, Jun 26, 2019 at 03:18:07PM -0600, Logan Gunthorpe wrote:
>>> I don't think we should make drives do that. What if it got CMB memory
>>> on some other device?
>> Huh? A driver submitting P2P requests finds appropriate memory to use
>> based on the DMA device that will be doing the mapping. It *has* to. It
>> doesn't necessarily have control over which P2P provider it might find
>> (ie. it may get CMB memory from a random NVMe device), but it easily
>> knows the NVMe device it got the CMB memory for. Look at the existing
>> code in the nvme target.
> No, this all thinking about things from the CMB perspective. With CMB
> you don't care about the BAR location because it is just a temporary
> buffer. That is a unique use model.
> Every other case has data residing in BAR memory that can really only
> reside in that one place (ie on a GPU/FPGA DRAM or something). When an IO
> against that is run it should succeed, even if that means bounce
> buffering the IO - as the user has really asked for this transfer to
> happen.
> We certainly don't get to generally pick where the data resides before
> starting the IO, that luxury is only for CMB.

I disagree. If we we're going to implement a "bounce" we'd probably want
to do it in two DMA requests. So the GPU/FPGA driver would first decide
whether it can do it P2P directly and, if it can't, would want to submit
a DMA request copy the data to host memory and then submit an IO
normally to the data's final destination.

I think it would be a larger layering violation to have the NVMe driver
(for example) memcpy data off a GPU's bar during a dma_map step to
support this bouncing. And it's even crazier to expect a DMA transfer to
be setup in the map step.