linux-kernel - Re: [RFC] Make use of non-dynamic dmabuf in RDMA

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Date:   Tue, 24 Aug 2021 12:06:00 +0300
From:   Gal Pressman <galpress@...zon.com>
To:     Christian König <christian.koenig@....com>,
        Jason Gunthorpe <jgg@...pe.ca>
CC:     Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Doug Ledford <dledford@...hat.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Oded Gabbay <ogabbay@...ana.ai>,
        Tomer Tayar <ttayar@...ana.ai>,
        Yossi Leybovich <sleybo@...zon.com>,
        Alexander Matushevsky <matua@...zon.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Jianxin Xiong <jianxin.xiong@...el.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC] Make use of non-dynamic dmabuf in RDMA

On 23/08/2021 13:43, Christian König wrote:
> Am 21.08.21 um 11:16 schrieb Gal Pressman:
>> On 20/08/2021 17:32, Jason Gunthorpe wrote:
>>> On Fri, Aug 20, 2021 at 03:58:33PM +0300, Gal Pressman wrote:
>>>
>>>> Though it would've been nicer if we could agree on a solution that could work
>>>> for more than 1-2 RDMA devices, using the existing tools the RDMA subsystem
>>>> has.
>>> I don't think it can really be done, revoke is necessary, and isn't a
>>> primitive we have today.
>>>
>>> Revoke is sort of like rereg MR, but with a guaranteed no-change to
>>> the lkey/rkey
>>>
>>> Then there is the locking complexity of linking the mr creation and
>>> destruction to the lifecycle of the pages, which is messy and maybe
>>> not general. For instance mlx5 would call its revoke_mr, disconnect
>>> the dmabuf then destroy the mkey - but this is only safe because mlx5
>>> HW can handle concurrent revokes.
>> Thanks, that makes sense.
>>
>>>> That's why I tried to approach this by denying such attachments for non-ODP
>>>> importers instead of exposing a "limited" dynamic importer.
>>> That is fine if there is no revoke - once revoke exists we must have
>>> driver and HW support.
>> Agree.
>> IIUC, we're talking about three different exporter "types":
>> - Dynamic with move_notify (requires ODP)
>> - Dynamic with revoke_notify
>> - Static
>>
>> Which changes do we need to make the third one work?
> 
> Basically none at all in the framework.
> 
> You just need to properly use the dma_buf_pin() function when you start using a
> buffer (e.g. before you create an attachment) and the dma_buf_unpin() function
> after you are done with the DMA-buf.

I replied to your previous mail, but I'll ask again.
Doesn't the pin operation migrate the memory to host memory?