linux-kernel - Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

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Date:   Thu, 17 Jan 2019 10:30:01 +0100
From:   "hch@....de" <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     "hch@....de" <hch@....de>,
        Thomas Hellstrom <thellstrom@...are.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yong.zhi@...el.com" <yong.zhi@...el.com>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "syeh@...are.com" <syeh@...are.com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "bingbu.cao@...el.com" <bingbu.cao@...el.com>,
        "imre.deak@...el.com" <imre.deak@...el.com>,
        "tian.shu.qiu@...el.com" <tian.shu.qiu@...el.com>,
        "jian.xu.zheng@...el.com" <jian.xu.zheng@...el.com>,
        "shiraz.saleem@...el.com" <shiraz.saleem@...el.com>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

On Wed, Jan 16, 2019 at 10:24:36AM -0700, Jason Gunthorpe wrote:
> The fact is there is 0 industry interest in using RDMA on platforms
> that can't do HW DMA cache coherency - the kernel syscalls required to
> do the cache flushing on the IO path would just destroy performance to
> the point of making RDMA pointless. Better to use netdev on those
> platforms.

In general there is no syscall required for doing cache flushing, you
just issue the proper instructions directly from userspace. 

> The reality is that *all* the subsytems doing DMA kernel bypass are
> ignoring the DMA mapping rules, I think we should support this better,
> and just accept that user space DMA will not be using syncing. Block
> access in cases when this is required, otherwise let it work as is
> today.

In that case we just need to block userspace DMA access entirely.
Which given the amount of problems it creates sounds like a pretty
good idea anyway.