Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [RFC] Make use of non-dynamic dmabuf in RDMA
To:     Jason Gunthorpe <jgg@ziepe.ca>
CC:     Daniel Vetter <daniel@ffwll.ch>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Doug Ledford <dledford@redhat.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-rdma <linux-rdma@vger.kernel.org>,
        Oded Gabbay <ogabbay@habana.ai>,
        Tomer Tayar <ttayar@habana.ai>,
        Yossi Leybovich <sleybo@amazon.com>,
        Alexander Matushevsky <matua@amazon.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Jianxin Xiong <jianxin.xiong@intel.com>,
        John Hubbard <jhubbard@nvidia.com>
References: <20210818074352.29950-1-galpress@amazon.com>
 <CAKMK7uGZ_eX+XfYJU6EkKEOVrHz3q6QMxaEbyyD3_1iqj9YSjw@mail.gmail.com>
 <20210819230602.GU543798@ziepe.ca>
 <CAKMK7uGgQWcs4Va6TGN9akHSSkmTs1i0Kx+6WpeiXWhJKpasLA@mail.gmail.com>
 <20210820123316.GV543798@ziepe.ca>
 <0fc94ac0-2bb9-4835-62b8-ea14f85fe512@amazon.com>
 <20210820143248.GX543798@ziepe.ca>
From:   Gal Pressman <galpress@amazon.com>
Message-ID: <da6364b7-9621-a384-23b0-9aa88ae232e5@amazon.com>
Date:   Sat, 21 Aug 2021 12:16:08 +0300
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0)
 Gecko/20100101 Thunderbird/78.13.0
MIME-Version: 1.0
In-Reply-To: <20210820143248.GX543798@ziepe.ca>
Content-Type: text/plain; charset="utf-8"
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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?