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

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Message-ID: <da6364b7-9621-a384-23b0-9aa88ae232e5@amazon.com>
Date:   Sat, 21 Aug 2021 12:16:08 +0300
From:   Gal Pressman <galpress@...zon.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
CC:     Daniel Vetter <daniel@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        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 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?