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

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Date:   Thu, 17 Jan 2019 10:47:38 +0000
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     "hch@....de" <hch@....de>, "jgg@...pe.ca" <jgg@...pe.ca>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "syeh@...are.com" <syeh@...are.com>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "yong.zhi@...el.com" <yong.zhi@...el.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "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 Thu, 2019-01-17 at 10:30 +0100, hch@....de wrote:
> 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.

But what if there are other coherence issues? Like bounce-buffers?
I'd like to +1 on what Jason says about industry interest: FWIW, vmwgfx
is probably one of the graphics drivers that would lend itself best to
do a fully-dma-interface compliant graphics stack experiment. But being
a paravirtual driver, all platforms we can ever run on are fully
coherent unless someone introduces a fake incoherency by forcing
swiotlb. Putting many man-months of effort into supporting systems on
which we would never run on and can never test on can never make more
than academic sense.

>  
> 
> > 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.

I'm not sure I'm following you here. Are you suggesting scratching
support for all (GP)GPU- and RDMA drivers?

Thanks,
Thomas