Re: [PATCH v1] arm64: errata: Workaround NVIDIA Olympus device store/load ordering erratum

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jun 04, 2026 at 06:12:54PM -0500, Shanker Donthineni wrote:
> On systems with NVIDIA Olympus cores, a Device-nGnR* load can be
> observed by a peripheral before an older, non-overlapping Device-nGnR*
> store to the same peripheral. This breaks the program-order guarantee
> that software expects for Device-nGnR* accesses and can leave a
> peripheral in an incorrect state, as a load is observed before an
> earlier store takes effect.
> 
> The erratum can occur only when all of the following apply:
> 
>   - A PE executes a Device-nGnR* store followed by a younger
>     Device-nGnR* load.
>   - The store is not a store-release.
>   - The accesses target the same peripheral and do not overlap in bytes.
>   - There is at most one intervening Device-nGnR* store in program
>     order, and there are no intervening Device-nGnR* loads.
>   - There is no DSB, and no DMB that orders loads, between the store and
>     the load.
>   - Specific micro-architectural and timing conditions occur.
> 
> Two ways to restore ordering: insert a barrier (any DSB, or a DMB that
> orders loads) between the store and the load, or make the store a
> store-release. A load-acquire on the load side would not help, because
> acquire semantics do not prevent a load from being observed ahead of an
> older store; only the store side (release or a barrier) closes the
> window.

Ignoring Device-nGnR*, a store-release followed by a load (not
load-acquire) would not guarantee any ordering. I assume the
store-release behaviour is specific to this erratum - part of the
preconditions.

The patch looks fine to me.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>