Re: [PATCH] iommu/arm-smmu-v3: Allocate cmdq_batch on the heap

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Mar 13, 2026 at 02:24:09AM +0800, Cheng-Yang Chou wrote:
> On Wed, Mar 11, 2026 at 02:22:50PM +0000, Pranjal Shrivastava wrote:
> > IMO, if we really want to address these, instead of kmalloc, we could
> > potentially consider some pre-allocated per-CPU buffers (that's a lot of
> > additional book-keeping though) to keep the data off the stack or
> > something similar following a simple rule: The fast path must be 
> > deterministic- no SLAB allocations and no introducing new failure points

> To resolve the stack warnings, I'm considering using per-CPU buffers in v2. 
> Does this direction sound reasonable, or would you prefer to keep it as-is
> to avoid the added complexity?

I don't think per-CPU buffers would work here either..

arm_smmu_atc_inv_master() is used in a preemptible context, while
arm_smmu_atc_inv_domain() can be called from an irq context.

Think of a !SMP case for simplification: we only have one per-CPU
buffer, which is not enough if an IRQ preempts the task context.

Maybe having a smaller backup array on the stack that can be used
when the heap allocation fails? Still, I don't see how to address
it elegantly without losing some of the performance optimization.

Nicolin