Re: [PATCH RFC v6 05/26] nova-core: mm: Add support to use PRAMIN windows to write to VRAM

Joel Fernandes
2026-01-22T23:16:06Z

On Wed, 21 Jan 2026 12:52:10 -0500, Joel Fernandes wrote:
> I think we can incrementally build on this series to add support for the same,
> it is not something this series directly addresses since I have spend majority
> of my time last several months making translation *work* which is itself no east
> task. This series is just preliminary based on work from last several months and
> to make BAR1 work. For instance, I kept PRAMIN simple based on feedback that we
> don't want to over complicate without fully understanding all the requirements.
> There is also additional requirements for locking design that have implications
> with DMA fencing etc, for instance.
>
> Anyway thinking out loud, I am thinking for handling concurrency at the page
> table entry level (if we ever need it), we could use per-PT spinlocks similar to
> the Linux kernel. But lets plan on how to do this properly and based on actual
> requirements.

Thanks for the discussion on concurrency, Zhi.

My plan is to make TLB and PRAMIN use immutable references in their function
calls and then implement internal locking. I've already done this for the GPU
buddy functions, so it should be doable, and we'll keep it consistent. As a
result, we will have finer-grain locking on the memory management objects
instead of requiring to globally lock a common GpuMm object. I'll plan on
doing this for v7.

Also, the PTE allocation race you mentioned is already handled by PRAMIN
serialization. Since threads must hold the PRAMIN lock to write page table
entries, concurrent writers are not possible:

Thread A: acquire PRAMIN lock
Thread A: read PDE (via PRAMIN) -> NULL
Thread A: alloc PT page, write PDE
Thread A: release PRAMIN lock

Thread B: acquire PRAMIN lock
Thread B: read PDE (via PRAMIN) -> sees A's pointer
Thread B: uses existing PT page, no allocation needed

No atomic compare-and-swap on VRAM is needed because the PRAMIN lock serializes
access. Please let me know if you had a different scenario in mind, but I think
this covers it.

Zhi, feel free to use v6 though for any testing you are doing while I
rework the locking.

--
Joel Fernandes