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 3:15 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Jun 22, 2021 at 03:04:30PM +0300, Oded Gabbay wrote:
> > On Tue, Jun 22, 2021 at 3:01 PM Jason Gunthorpe <jgg@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@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 ?
>
> 1) Setting sg_page to NULL
> 2) 'mapping' pages for P2P DMA without going through the iommu
> 3) Allowing P2P DMA without using the p2p dma API to validate that it
>    can work at all in the first place.
>
> All of these result in functional bugs in certain system
> configurations.
>
> Jason

Hi Jason,
Thanks for the feedback.
Regarding point 1, why is that a problem if we disable the option to
mmap the dma-buf from user-space ? We don't want to support CPU
fallback/Direct IO.
In addition, I didn't see any problem with sg_page being NULL in the
RDMA p2p dma-buf code. Did I miss something here ?

Regarding points 2 & 3, I want to examine them more closely in a KVM
virtual machine environment with IOMMU enabled.
I will take two GAUDI devices and use one as an exporter and one as an
importer. I want to see that the solution works end-to-end, with real
device DMA from importer to exporter.
I fear that the dummy importer I wrote is bypassing these two issues
you brought up.

So thanks again and I'll get back and update once I've finished testing it.

Oded