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Subject: Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export
 FD for DMA-BUF
To:     Jason Gunthorpe <jgg@ziepe.ca>, Oded Gabbay <oded.gabbay@gmail.com>
Cc:     Gal Pressman <galpress@amazon.com>, sleybo@amazon.com,
        linux-rdma <linux-rdma@vger.kernel.org>,
        Oded Gabbay <ogabbay@kernel.org>,
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        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        Doug Ledford <dledford@redhat.com>,
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        Greg KH <gregkh@linuxfoundation.org>,
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        Leon Romanovsky <leonro@nvidia.com>,
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
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Date:   Tue, 22 Jun 2021 08:37:15 +0200
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Am 22.06.21 um 01:29 schrieb Jason Gunthorpe:
> 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.

The major problem with this approach is that DMA-buf is also used for 
memory which isn't CPU accessible.

That was one of the reasons we didn't even considered using the mapping 
memory approach for GPUs.

Regards,
Christian.

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