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

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Message-ID: <20211210124204.GG6467@ziepe.ca>
Date:   Fri, 10 Dec 2021 08:42:04 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Shunsuke Mie <mie@...l.co.jp>
Cc:     Zhu Yanjun <zyjzyj2000@...il.com>,
        Christian König <christian.koenig@....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 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?

Jason