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

From: Logan Gunthorpe
Date: Mon Jun 24 2019 - 12:10:27 EST

On 2019-06-24 7:46 a.m., Jason Gunthorpe wrote:
> On Mon, Jun 24, 2019 at 09:31:26AM +0200, Christoph Hellwig wrote:
>> On Thu, Jun 20, 2019 at 04:33:53PM -0300, Jason Gunthorpe wrote:
>>>> My primary concern with this is that ascribes a level of generality
>>>> that just isn't there for peer-to-peer dma operations. "Peer"
>>>> addresses are not "DMA" addresses, and the rules about what can and
>>>> can't do peer-DMA are not generically known to the block layer.
>>> ?? The P2P infrastructure produces a DMA bus address for the
>>> initiating device that is is absolutely a DMA address. There is some
>>> intermediate CPU centric representation, but after mapping it is the
>>> same as any other DMA bus address.
>>> The map function can tell if the device pair combination can do p2p or
>>> not.
>> At the PCIe level there is no such thing as a DMA address, it all
>> is bus address with MMIO and DMA in the same address space (without
>> that P2P would have not chance of actually working obviously). But
>> that bus address space is different per "bus" (which would be an
>> root port in PCIe), and we need to be careful about that.
> Sure, that is how dma_addr_t is supposed to work - it is always a
> device specific value that can be used only by the device that it was
> created for, and different devices could have different dma_addr_t
> values for the same memory.
> So when Logan goes and puts dma_addr_t into the block stack he must
> also invert things so that the DMA map happens at the start of the
> process to create the right dma_addr_t early.

Yes, that's correct. The intent was to invert it so the dma_map could
happen at the start of the process so that P2PDMA code could be called
with all the information it needs to make it's decision on how to map;
without having to hook into the mapping process of every driver that
wants to participate.