Re: [RFC PATCH v2 0/3] RDMA: add dma-buf support

[Alex Deucher <alexdeucher@gmail.com>]

On Wed, Jul 8, 2020 at 10:20 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 08.07.20 um 11:49 schrieb Daniel Vetter:
> > On Wed, Jul 08, 2020 at 11:38:31AM +0200, Christian König wrote:
> >> Am 07.07.20 um 23:58 schrieb Xiong, Jianxin:
> >>>> -----Original Message-----
> >>>> From: Christian König <christian.koenig@amd.com>
> >>>> Am 03.07.20 um 15:14 schrieb Jason Gunthorpe:
> >>>>> On Fri, Jul 03, 2020 at 02:52:03PM +0200, Daniel Vetter wrote:
> >>>>>
> >>>>>> So maybe I'm just totally confused about the rdma model. I thought:
> >>>>>> - you bind a pile of memory for various transactions, that might
> >>>>>> happen whenever. Kernel driver doesn't have much if any insight into
> >>>>>> when memory isn't needed anymore. I think in the rdma world that's
> >>>>>> called registering memory, but not sure.
> >>>>> Sure, but once registered the memory is able to be used at any moment
> >>>>> with no visibilty from the kernel.
> >>>>>
> >>>>> Unlike GPU the transactions that trigger memory access do not go
> >>>>> through the kernel - so there is no ability to interrupt a command
> >>>>> flow and fiddle with mappings.
> >>>> This is the same for GPUs with user space queues as well.
> >>>>
> >>>> But we can still say for a process if that this process is using a DMA-buf which is moved out and so can't run any more unless the DMA-buf is
> >>>> accessible again.
> >>>>
> >>>> In other words you somehow need to make sure that the hardware is not accessing a piece of memory any more when you want to move it.
> >>>>
> >>> While a process can be easily suspended, there is no way to tell the RDMA NIC not to process posted work requests that use specific memory regions (or with any other conditions).
> >>>
> >>> So far it appears to me that DMA-buf dynamic mapping for RDMA is only viable with ODP support. For NICs without ODP, a way to allow pinning the device memory is still needed.
> >> And that's exactly the reason why I introduced explicit pin()/unpin()
> >> functions into the DMA-buf API:
> >> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Flatest%2Fsource%2Fdrivers%2Fdma-buf%2Fdma-buf.c%23L811&amp;data=02%7C01%7Cchristian.koenig%40amd.com%7C6d785861acc542a2f53608d823243a7c%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637297985792135311&amp;sdata=bBrkDynlACE9DAIlGntxXhE1unr%2FBxw5IRTm6AtV6WQ%3D&amp;reserved=0
> >>
> >> It's just that at least our devices drivers currently prevent P2P with
> >> pinned DMA-buf's for two main reasons:
> >>
> >> a) To prevent deny of service attacks because P2P BARs are a rather rare
> >> resource.
> >>
> >> b) To prevent failures in configuration where P2P is not always possible
> >> between all devices which want to access a buffer.
> > So the above is more or less the question in the cover letter (which
> > didn't make it to dri-devel). Can we somehow throw that limitation out, or
> > is that simply not a good idea?
>
> At least for the AMD graphics drivers that's most certain not a good idea.
>
> We do have an use case where buffers need to be in system memory because
> P2P doesn't work.
>
> And by pinning them to VRAM you can create a really nice deny of service
> attack against the X system.
>

On the other hand, on modern platforms with large or resizable BARs,
you may end up with systems with more vram than system ram.

Alex


> > Simply moving buffers to system memory when they're pinned does simplify a
> > lot of headaches. For a specific custom built system we can avoid that
> > maybe, but I think upstream is kinda a different thing.
>
> Yes, agree completely on that. Customers which are willing to take the
> risk can easily do this themselves.
>
> But that is not something we should probably do for upstream.
>
> Regards,
> Christian.
>
> >
> > Cheers, Daniel
> >
> >> Regards,
> >> Christian.
> >>
> >>> Jianxin
> >>>
> >>>> Christian.
> >>>>
> >>>>> Jason
>
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