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

[Oded Gabbay <oded.gabbay@gmail.com>]

On Tue, Jun 22, 2021 at 9:37 AM Christian König
<ckoenig.leichtzumerken@gmail.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
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