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

[Oded Gabbay]

On Tue, Jun 22, 2021 at 3:01 PM Jason Gunthorpe <jgg at ziepe.ca> wrote:
>
> On Tue, Jun 22, 2021 at 11:42:27AM +0300, Oded Gabbay wrote:
> > On Tue, Jun 22, 2021 at 9:37 AM Christian König
> > <ckoenig.leichtzumerken at 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 isn't an issue here because the memory is only intended to be
> used with P2P transfers so it must be CPU accessible.
>
> > > That was one of the reasons we didn't even considered using the mapping
> > > memory approach for GPUs.
>
> Well, now we have DEVICE_PRIVATE memory that can meet this need
> too.. Just nobody has wired it up to hmm_range_fault()
>
> > > > 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.
>
> You still have the issue that this patch is doing all of this P2P
> stuff wrong - following the already NAK'd AMD approach.

Could you please point me exactly to the lines of code that are wrong
in your opinion ?
I find it hard to understand from your statement what exactly you
think that we are doing wrong.
The implementation is found in the second patch in this patch-set.

Thanks,
Oded
>
> > 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.
>
> It is trivial to get the struct page for a PCI BAR.
>
> Jason