Re: [PATCH v4 4/4] gpu: nova-core: fix stack overflow in GSP memory allocation

["Alexandre Courbot" <acourbot@nvidia.com>]

On Tue Mar 10, 2026 at 1:34 AM JST, Tim Kovalenko via B4 Relay wrote:
> From: Tim Kovalenko <tim.kovalenko@proton.me>
>
> The `Cmdq::new` function was allocating a `PteArray` struct on the stack
> and was causing a stack overflow with 8216 bytes.
>
> Modify the `PteArray` to calculate and write the Page Table Entries
> directly into the coherent DMA buffer one-by-one. This reduces the stack
> usage quite a lot.
>
> Signed-off-by: Tim Kovalenko <tim.kovalenko@proton.me>
> ---
>  drivers/gpu/nova-core/gsp.rs      | 34 +++++++++++++++++++---------------
>  drivers/gpu/nova-core/gsp/cmdq.rs | 15 ++++++++++++++-
>  2 files changed, 33 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/gsp.rs b/drivers/gpu/nova-core/gsp.rs
> index 25cd48514c777cb405a2af0acf57196b2e2e7837..20170e483e04c476efce8997b3916b0ad829ed38 100644
> --- a/drivers/gpu/nova-core/gsp.rs
> +++ b/drivers/gpu/nova-core/gsp.rs
> @@ -47,16 +47,11 @@
>  unsafe impl<const NUM_ENTRIES: usize> AsBytes for PteArray<NUM_ENTRIES> {}
>  
>  impl<const NUM_PAGES: usize> PteArray<NUM_PAGES> {
> -    /// Creates a new page table array mapping `NUM_PAGES` GSP pages starting at address `start`.
> -    fn new(start: DmaAddress) -> Result<Self> {
> -        let mut ptes = [0u64; NUM_PAGES];
> -        for (i, pte) in ptes.iter_mut().enumerate() {
> -            *pte = start
> -                .checked_add(num::usize_as_u64(i) << GSP_PAGE_SHIFT)
> -                .ok_or(EOVERFLOW)?;
> -        }
> -
> -        Ok(Self(ptes))
> +    /// Returns the page table entry for `index`, for a mapping starting at `start` DmaAddress.
> +    fn entry(start: DmaAddress, index: usize) -> Result<u64> {
> +        start
> +            .checked_add(num::usize_as_u64(index) << GSP_PAGE_SHIFT)
> +            .ok_or(EOVERFLOW)
>      }
>  }
>  
> @@ -86,16 +81,25 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
>              NUM_PAGES * GSP_PAGE_SIZE,
>              GFP_KERNEL | __GFP_ZERO,
>          )?);
> -        let ptes = PteArray::<NUM_PAGES>::new(obj.0.dma_handle())?;
> +
> +        let start_addr = obj.0.dma_handle();
>  
>          // SAFETY: `obj` has just been created and we are its sole user.
> -        unsafe {
> -            // Copy the self-mapping PTE at the expected location.
> +        let pte_region = unsafe {
>              obj.0
> -                .as_slice_mut(size_of::<u64>(), size_of_val(&ptes))?
> -                .copy_from_slice(ptes.as_bytes())
> +                .as_slice_mut(size_of::<u64>(), NUM_PAGES * size_of::<u64>())?
>          };
>  
> +        // This is a  one by one GSP Page write to the memory
> +        // to avoid stack overflow when allocating the whole array at once.
> +        for (i, chunk) in pte_region.chunks_exact_mut(size_of::<u64>()).enumerate() {
> +            let pte_value = start_addr
> +                .checked_add(num::usize_as_u64(i) << GSP_PAGE_SHIFT)
> +                .ok_or(EOVERFLOW)?;
> +
> +            chunk.copy_from_slice(&pte_value.to_ne_bytes());
> +        }
> +
>          Ok(obj)
>      }
>  }
> diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
> index 0056bfbf0a44cfbc5a0ca08d069f881b877e1edc..c8327d3098f73f9b880eee99038ad10a16e1e32d 100644
> --- a/drivers/gpu/nova-core/gsp/cmdq.rs
> +++ b/drivers/gpu/nova-core/gsp/cmdq.rs
> @@ -202,7 +202,20 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
>  
>          let gsp_mem =
>              CoherentAllocation::<GspMem>::alloc_coherent(dev, 1, GFP_KERNEL | __GFP_ZERO)?;
> -        dma_write!(gsp_mem, [0]?.ptes, PteArray::new(gsp_mem.dma_handle())?);
> +
> +        const NUM_PTES: usize = GSP_PAGE_SIZE / size_of::<u64>();
> +
> +        let start = gsp_mem.dma_handle();
> +        // One by one GSP Page write to the memory to avoid stack overflow when allocating
> +        // the whole array at once.
> +        for i in 0..NUM_PTES {
> +            dma_write!(
> +                gsp_mem,
> +                [0]?.ptes.0[i],
> +                PteArray::<NUM_PTES>::entry(start, i)?

Does `::<NUM_PTES>` need to be mentioned here, or is the compiler able
to infer it?

In any case, the updated patch

Acked-by: Alexandre Courbot <acourbot@nvidia.com>

Thanks!