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

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Message-ID: <fa124990-ee0c-7401-019e-08109e338042@amd.com>
Date:   Mon, 23 Aug 2021 12:43:10 +0200
From:   Christian König <christian.koenig@....com>
To:     Gal Pressman <galpress@...zon.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

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.

Regards,
Christian.