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

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Date:   Tue, 14 Dec 2021 09:53:46 +0100
From:   Christian König <christian.koenig@....com>
To:     Shunsuke Mie <mie@...l.co.jp>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, 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 13.12.21 um 12:18 schrieb Shunsuke Mie:
> 2021年12月10日(金) 22:29 Christian König <christian.koenig@....com>:
>> Am 10.12.21 um 14:26 schrieb Jason Gunthorpe:
>>> 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.
>>>
>>> ??
>> Well, yes that can go boom pretty quickly.
> Sorry, I was wrong. The dma_buf_map treats both iomem and vaddr region, but
> this RFC only supports vaddr. Advantage of the partial support is we can use the
> vaddr dma-buf in RXE without changing a rxe data copy implementation.

Well that is most likely not a good idea.

For example buffers for GPU drivers can be placed in both MMIO memory 
and system memory.

If you don't want to provoke random failures you *MUST* be able to 
handle both if you want to use this.

Regards,
Christian.

>
> An example of a dma-buf pointing to a vaddr is some gpu drivers use RAM for
> VRAM and we can get dma-buf for the region that indicates vaddr regions.
> Specifically, the gpu driver using gpu/drm/drm_gem_cma_helper.c is one such
> example.
>
>> Not sure what they want to use this for.
> I'd like to use RDMA with RXE for that memory region.
>
> Best,
> Shunsuke
>> Christian.
>>
>>> Jason