Re: [PATCH 01/12] dma-buf: add dynamic caching of sg_table

[Daniel Vetter]

On Thu, May 23, 2019 at 1:21 PM Koenig, Christian
<Christian.Koenig@xxxxxxx> wrote:
>
> Am 22.05.19 um 20:30 schrieb Daniel Vetter:
> > [SNIP]
> >> Well, it seems you are making incorrect assumptions about the cache
> >> maintenance of DMA-buf here.
> >>
> >> At least for all DRM devices I'm aware of mapping/unmapping an
> >> attachment does *NOT* have any cache maintenance implications.
> >>
> >> E.g. the use case you describe above would certainly fail with amdgpu,
> >> radeon, nouveau and i915 because mapping a DMA-buf doesn't stop the
> >> exporter from reading/writing to that buffer (just the opposite actually).
> >>
> >> All of them assume perfectly coherent access to the underlying memory.
> >> As far as I know there is no documented cache maintenance requirements
> >> for DMA-buf.
> > I think it is documented. It's just that on x86, we ignore that
> > because the dma-api pretends there's never a need for cache flushing
> > on x86, and that everything snoops the cpu caches. Which isn't true
> > since over 20 ago when AGP happened. The actual rules for x86 dma-buf
> > are very much ad-hoc (and we occasionally reapply some duct-tape when
> > cacheline noise shows up somewhere).
>
> Well I strongly disagree on this. Even on x86 at least AMD GPUs are also
> not fully coherent.
>
> For example you have the texture cache and the HDP read/write cache. So
> when both amdgpu as well as i915 would write to the same buffer at the
> same time we would get a corrupted data as well.
>
> The key point is that it is NOT DMA-buf in it's map/unmap call who is
> defining the coherency, but rather the reservation object and its
> attached dma_fence instances.
>
> So for example as long as a exclusive reservation object fence is still
> not signaled I can't assume that all caches are flushed and so can't
> start with my own operation/access to the data in question.

The dma-api doesn't flush device caches, ever. It might flush some
iommu caches or some other bus cache somewhere in-between. So it also
won't ever make sure that multiple devices don't trample on each
another. For that you need something else (like reservation object,
but I think that's not really followed outside of drm much).

The other bit is the coherent vs. non-coherent thing, which in the
dma-api land just talks about whether cpu/device access need extra
flushing or not. Now in practice that extra flushing is always only
cpu side, i.e. will cpu writes/reads go through the cpu cache, and
will device reads/writes snoop the cpu caches. That's (afaik at least,
an in practice, not the abstract spec) the _only_ thing dma-api's
cache maintenance does. For 0 copy that's all completely irrelevant,
because as soon as you pick a mode where you need to do manual cache
management you've screwed up, it's not 0-copy anymore really.

The other hilarious stuff is that on x86 we let userspace (at least
with i915) do that cache management, so the kernel doesn't even have a
clue. I think what we need in dma-buf (and dma-api people will scream
about the "abstraction leak") is some notition about whether an
importer should snoop or not (or if that device always uses non-snoop
or snooped transactions). But that would shred the illusion the
dma-api tries to keep up that all that matters is whether a mapping is
coherent from the cpu's pov or not, and you can achieve coherence both
with a cache cpu mapping + snooped transactions, or with wc cpu side
and non-snooped transactions. Trying to add cache managment (which
some dma-buf exporter do indeed attempt to) will be even worse.

Again, none of this is about preventing concurrent writes, or making
sure device caches are flushed correctly around batches.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch