Re: [PATCH v2] arm64: errata: Workaround for SI L1 downstream coherency issue

[Kuan-Wei Chiu <visitorckw@gmail.com>]

Hi Lucas,

On Mon, Dec 29, 2025 at 03:36:19AM +0000, Lucas Wei wrote:
> When software issues a Cache Maintenance Operation (CMO) targeting a
> dirty cache line, the CPU and DSU cluster may optimize the operation by
> combining the CopyBack Write and CMO into a single combined CopyBack
> Write plus CMO transaction presented to the interconnect (MCN).
> For these combined transactions, the MCN splits the operation into two
> separate transactions, one Write and one CMO, and then propagates the
> write and optionally the CMO to the downstream memory system or external
> Point of Serialization (PoS).
> However, the MCN may return an early CompCMO response to the DSU cluster
> before the corresponding Write and CMO transactions have completed at
> the external PoS or downstream memory. As a result, stale data may be
> observed by external observers that are directly connected to the
> external PoS or downstream memory.
> 
> This erratum affects any system topology in which the following
> conditions apply:
>  - The Point of Serialization (PoS) is located downstream of the
>    interconnect.
>  - A downstream observer accesses memory directly, bypassing the
>    interconnect.
> 
> Conditions:
> This erratum occurs only when all of the following conditions are met:
>  1. Software executes a data cache maintenance operation, specifically,
>     a clean or invalidate by virtual address (DC CVAC, DC CIVAC, or DC
>     IVAC), that hits on unique dirty data in the CPU or DSU cache. This
>     results in a combined CopyBack and CMO being issued to the
>     interconnect.
>  2. The interconnect splits the combined transaction into separate Write
>     and CMO transactions and returns an early completion response to the
>     CPU or DSU before the write has completed at the downstream memory
>     or PoS.
>  3. A downstream observer accesses the affected memory address after the
>     early completion response is issued but before the actual memory
>     write has completed. This allows the observer to read stale data
>     that has not yet been updated at the PoS or downstream memory.
> 
> The implementation of workaround put a second loop of CMOs at the same
> virtual address whose operation meet erratum conditions to wait until
> cache data be cleaned to PoC.. This way of implementation mitigates
> performance panalty compared to purly duplicate orignial CMO.
> 
> Reported-by: kernel test robot <lkp@intel.com>

I assume the Reported-by tag was added due to the sparse warning in v1?
Since this patch fixes a hardware erratum rather than an issue reported
by the robot, I don't think we need this tag here.

Generally, we don't add Reported-by for fixing robot warnings across
patch versions.

Regards,
Kuan-Wei