Re: [PATCH 10/11] gpu: nova-core: LibosMemoryRegionInitArgument size must be page aligned

[Timur Tabi <ttabi@nvidia.com>]

On Fri, 2025-12-05 at 09:35 +0900, Alexandre Courbot wrote:
> 
> With one caveat: `new` now returns a 4K object on the stack, which we
> definitely want to avoid. So maybe we can have a wrapper for things we
> want to align the 4K:
> 
>   #[repr(C)]
>   pub(crate) struct GspPageAligned<T> {
>     pub(crate) inner: T,
>     padding: [u8; GSP_PAGE_SIZE - core::mem::size_of::<T>()],
>   }
> 
> We would then allocate the CoherentAllocation using a
> `GspPageAligned<GspArgumentsCached>`, and initialize its useful data
> with:
> 
>   dma_write!(rmargs[0].inner = fw::GspArgumentsCached::new(&cmdq))?;

I'm confused.  Aren't we already avoiding the stack?  This is the code today:

        let rmargs = CoherentAllocation::<GspArgumentsCached>::alloc_coherent(
            dev,
            1,
            GFP_KERNEL | __GFP_ZERO,
        )?;
        dma_write!(rmargs[0] = fw::GspArgumentsCached::new(&cmdq))?;

The only difference with what's there today vs what you suggest is the ".inner", and I think I can
avoid even that if I make GspPageAligned a tuple instead of a named struct.

> 
> > I had to remove the #[repr(transparent)].  Is that
> > okay?  The code compiles and seems to work.
> 
> As long as the struct is `repr(C)`, the layout will be what we expect.
> Actually this made me realize that `repr(C)` is technically what we want for our
> bindings abstractions, not `repr(transparent)` - both happen to have the
> same effect since the wrapper struct is `repr(C)` anyway, but the latter
> is more restrictive than we need.
> 
> Glad we found an elegant way to address this!

Actually, I think a more elegant solution would be a new variant of
CoherentAllocation::alloc_coherent() that takes a size to allocate instead of using size_of::<T>.  

In fact, I wonder if it makes sense to always grow the size of the allocation to the nearest page,
since dma_alloc_attrs() always allocates whole pages anyway.  Perhaps CoherentAllocation<T> needs an
allocated() method in addition to a size() method.  size() returns count*sizeof as today, and
allocated() returns that value rounded up to the nearest PAGE_SIZE.