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

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Date:   Tue, 22 Jun 2021 11:42:27 +0300
From:   Oded Gabbay <oded.gabbay@...il.com>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Gal Pressman <galpress@...zon.com>,
        sleybo@...zon.com, linux-rdma <linux-rdma@...r.kernel.org>,
        Oded Gabbay <ogabbay@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Christian König <christian.koenig@....com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Doug Ledford <dledford@...hat.com>,
        Tomer Tayar <ttayar@...ana.ai>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alex Deucher <alexander.deucher@....com>,
        Leon Romanovsky <leonro@...dia.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export
 FD for DMA-BUF

On Tue, Jun 22, 2021 at 9:37 AM Christian König
<ckoenig.leichtzumerken@...il.com> wrote:
>
> 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..

Thanks Jason for the clarification, but I honestly prefer to use
DMA-BUF at the moment.
It gives us just what we need (even more than what we need as you
pointed out), it is *already* integrated and tested in the RDMA
subsystem, and I'm feeling comfortable using it as I'm somewhat
familiar with it from my AMD days.

I'll go and read Logan's patch-set to see if that will work for us in
the future. Please remember, as Daniel said, we don't have struct page
backing our device memory, so if that is a requirement to connect to
Logan's work, then I don't think we will want to do it at this point.

Thanks,
Oded

> >
> > Jason
> > _______________________________________________
> > Linaro-mm-sig mailing list
> > Linaro-mm-sig@...ts.linaro.org
> > https://lists.linaro.org/mailman/listinfo/linaro-mm-sig
>

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