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

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Date:   Fri, 10 Dec 2021 13:47:37 +0100
From:   Christian König <christian.koenig@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Shunsuke Mie <mie@...l.co.jp>
Cc:     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

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.

Christian.

>
> Jason