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

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Message-ID: <20210820143248.GX543798@ziepe.ca>
Date:   Fri, 20 Aug 2021 11:32:48 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Gal Pressman <galpress@...zon.com>
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 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.

> 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.

Jason