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

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Message-ID: <20190117155417.GA9629@ziepe.ca>
Date:   Thu, 17 Jan 2019 08:54:17 -0700
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "hch@....de" <hch@....de>
Cc:     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 Thu, Jan 17, 2019 at 10:30:01AM +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. 

At least on the ARM/MIPS systems I've worked with like this the cache
manipulation instructions are privileged and cannot be executed by
userspace. So the general case requires a syscall.

> 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 doubt there is any support for that idea...

Jason