linux-kernel - Re: [RFC PATCH v4 0/2] RDMA/rxe: Add dma-buf support

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Date:   Mon, 20 Dec 2021 10:31:27 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christian König <christian.koenig@....com>,
        Shunsuke Mie <mie@...l.co.jp>,
        Zhu Yanjun <zyjzyj2000@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Doug Ledford <dledford@...hat.com>,
        Jianxin Xiong <jianxin.xiong@...el.com>,
        Leon Romanovsky <leon@...nel.org>,
        Maor Gottlieb <maorg@...dia.com>,
        Sean Hefty <sean.hefty@...el.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org,
        linux-media@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Damian Hobson-Garcia <dhobsong@...l.co.jp>,
        Takanari Hayama <taki@...l.co.jp>,
        Tomohito Esaki <etom@...l.co.jp>
Subject: Re: [RFC PATCH v4 0/2] RDMA/rxe: Add dma-buf support

On Fri, Dec 10, 2021 at 09:26:56AM -0400, Jason Gunthorpe wrote:
> On Fri, Dec 10, 2021 at 01:47:37PM +0100, Christian König wrote:
> > Am 10.12.21 um 13:42 schrieb Jason Gunthorpe:
> > > On Fri, Dec 10, 2021 at 08:29:24PM +0900, Shunsuke Mie wrote:
> > > > Hi Jason,
> > > > Thank you for replying.
> > > > 
> > > > 2021年12月8日(水) 2:14 Jason Gunthorpe <jgg@...pe.ca>:
> > > > > On Fri, Dec 03, 2021 at 12:51:44PM +0900, Shunsuke Mie wrote:
> > > > > > Hi maintainers,
> > > > > > 
> > > > > > Could you please review this patch series?
> > > > > Why is it RFC?
> > > > > 
> > > > > I'm confused why this is useful?
> > > > > 
> > > > > This can't do copy from MMIO memory, so it shouldn't be compatible
> > > > > with things like Gaudi - does something prevent this?
> > > > I think if an export of the dma-buf supports vmap, CPU is able to access the
> > > > mmio memory.
> > > > 
> > > > Is it wrong? If this is wrong, there is no advantages this changes..
> > > I don't know what the dmabuf folks did, but yes, it is wrong.
> > > 
> > > IOMEM must be touched using only special accessors, some platforms
> > > crash if you don't do this. Even x86 will crash if you touch it with
> > > something like an XMM optimized memcpy.
> > > 
> > > Christian? If the vmap succeeds what rules must the caller use to
> > > access the memory?
> > 
> > See dma-buf-map.h and especially struct dma_buf_map.
> > 
> > MMIO memory is perfectly supported here and actually the most common case.
> 
> Okay that looks sane, but this rxe RFC seems to ignore this
> completely. It stuffs the vaddr directly into a umem which goes to all
> manner of places in the driver.
> 
> ??

dma_buf_map is fairly new and we haven't rolled it out consistently yet.
In the past 10 years we simply yolo'd this :-)

Just an explanation, not an excuse for new code to not use dma_buf_map
consistently now that we fixed this mistake.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch