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

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Date:   Mon, 24 Jun 2019 10:10:16 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-pci@...r.kernel.org, linux-rdma <linux-rdma@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [RFC PATCH 00/28] Removing struct page from P2PDMA



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.

Logan