Re: [Question] Are "device exclusive non-swap entries" / "SVM atomics in Nouveau" still getting used in practice?

[Simona Vetter <simona.vetter@ffwll.ch>]

On Thu, Jan 23, 2025 at 11:20:37AM +0100, David Hildenbrand wrote:
> Hi,
> 
> I keep finding issues in our implementation of "device exclusive non-swap
> entries", and the way it messes with mapcounts is disgusting.
> 
> As a reminder, what we do here is to replace a PTE pointing to an anonymous
> page by a "device exclusive non-swap entry".
> 
> As long as the original PTE is in place, the only CPU can access it, as soon
> as the "device exclusive non-swap entry" is in place, only the device can
> access it. Conversion back and forth is triggered by CPU / device faults.
> 
> I have fixes/reworks/simplifications for most things, but as there is only a
> "real" single user in-tree of make_device_exclusive():
> 
> 	drivers/gpu/drm/nouveau/nouveau_svm.c
> 
> to "support SVM atomics in Nouveau [1]"
> 
> naturally I am wondering: is this still a thing on actual hardware, or is it
> already stale on recent hardware and not really required anymore?
> 
> 
> [1] https://lore.kernel.org/linux-kernel//6621654.gmDyfcmpjF@nvdebian/T/

As long as you don't have a coherent interconnect it's needed. On intel
discrete device atomics require device memory, so they need full hmm
migration (and hence wont use this function even once we land intel gpu
svm code in upstream). On integrated the gpu is tied into the coherency
fabric, so there it's not needed.

I think the more fundamental question with both this function here and
with forced migration to device memory is that there's no guarantee it
will work out. At least that's my understanding. And for this gpu device
atomics without coherent interconnect idea to work, we'd need to be able
to guarantee that we can make any page device exclusive. So from my side I
have some pretty big question marks on this entire thing overall.

Cheers, Sima
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch