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

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Date:   Fri, 10 Dec 2021 09:26:56 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christian König <christian.koenig@....com>
Cc:     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 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.

??

Jason