Re: [PATCH v7 2/4] arm64: Add BBM Level 2 cpu feature

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 17/06/2025 10:51, Mikołaj Lenczewski wrote:
> The Break-Before-Make cpu feature supports multiple levels (levels 0-2),
> and this commit adds a dedicated BBML2 cpufeature to test against
> support for.
> 
> To support BBML2 in as wide a range of contexts as we can, we want not
> only the architectural guarantees that BBML2 makes, but additionally
> want BBML2 to not create TLB conflict aborts. Not causing aborts avoids
> us having to prove that no recursive faults can be induced in any path
> that uses BBML2, allowing its use for arbitrary kernel mappings.
> 
> This feature builds on the previous ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE,
> as all early cpus must support BBML2 for us to enable it (and any later
> cpus must also support it to be onlined).
> 
> Not onlining late cpus that do not support BBML2 is unavoidable, as we
> might currently be using BBML2 semantics for kernel memory regions. This
> could cause faults in the late cpus, and would be difficult to unwind,
> so let us avoid the case altogether.
> 
> Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@arm.com>

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>