linux-kernel - Re: [RFC PATCH 04/28] block: Never bounce dma-direct bios

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Message-ID: <20190620172347.GE19891@ziepe.ca>
Date:   Thu, 20 Jun 2019 14:23:47 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-pci@...r.kernel.org,
        linux-rdma@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@....de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [RFC PATCH 04/28] block: Never bounce dma-direct bios

On Thu, Jun 20, 2019 at 10:12:16AM -0600, Logan Gunthorpe wrote:
> It is expected the creator of the dma-direct bio will ensure the
> target device can access the DMA address it's creating bios for.
> It's also not possible to bounce a dma-direct bio seeing the block
> layer doesn't have any way to access the underlying data behind
> the DMA address.
> 
> Thus, never bounce dma-direct bios.

I wonder how feasible it would be to implement a 'dma vec' copy
from/to? 

That is about the only operation you could safely do on P2P BAR
memory. 

I wonder if a copy implementation could somehow query the iommu layer
to get a kmap of the memory pointed at by the dma address so we don't
need to carry struct page around?

Jason