linux-kernel - Re: [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

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Message-ID: <20210621232912.GK1096940@ziepe.ca>
Date:   Mon, 21 Jun 2021 20:29:12 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Oded Gabbay <oded.gabbay@...il.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Greg KH <gregkh@...uxfoundation.org>,
        Oded Gabbay <ogabbay@...nel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>, Doug Ledford <dledford@...hat.com>,
        "airlied@...il.com" <airlied@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Gal Pressman <galpress@...zon.com>, sleybo@...zon.com,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Tomer Tayar <ttayar@...ana.ai>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

On Mon, Jun 21, 2021 at 10:24:16PM +0300, Oded Gabbay wrote:

> Another thing I want to emphasize is that we are doing p2p only
> through the export/import of the FD. We do *not* allow the user to
> mmap the dma-buf as we do not support direct IO. So there is no access
> to these pages through the userspace.

Arguably mmaping the memory is a better choice, and is the direction
that Logan's series goes in. Here the use of DMABUF was specifically
designed to allow hitless revokation of the memory, which this isn't
even using.

So you are taking the hit of very limited hardware support and reduced
performance just to squeeze into DMABUF..

Jason