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

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Date:   Mon, 24 Jun 2019 10:46:41 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.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 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.

I'm not totally clear if this series did that inversion, if it didn't
then it should not be using the dma_addr_t label at all, or refering
to anything as a 'dma address' as it is just confusing.

BTW, it is not just offset right? It is possible that the IOMMU can
generate unique dma_addr_t values for each device?? Simple offset is
just something we saw in certain embedded cases, IIRC.

Jason