linux-kernel - Re: [PATCH v5 1/3] arm64: Add BBM Level 2 cpu feature

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Message-ID: <5e49377e-6a3e-4a48-94c9-db06f13cffe8@arm.com>
Date: Tue, 25 Mar 2025 11:06:37 +0000
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Mikołaj Lenczewski <miko.lenczewski@....com>,
 ryan.roberts@....com, yang@...amperecomputing.com, corbet@....net,
 catalin.marinas@....com, will@...nel.org, jean-philippe@...aro.org,
 robin.murphy@....com, joro@...tes.org, akpm@...ux-foundation.org,
 ardb@...nel.org, mark.rutland@....com, joey.gouly@....com, maz@...nel.org,
 james.morse@....com, broonie@...nel.org, oliver.upton@...ux.dev,
 baohua@...nel.org, david@...hat.com, ioworker0@...il.com, jgg@...pe.ca,
 nicolinc@...dia.com, mshavit@...gle.com, jsnitsel@...hat.com,
 smostafa@...gle.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 iommu@...ts.linux.dev
Subject: Re: [PATCH v5 1/3] arm64: Add BBM Level 2 cpu feature

On 25/03/2025 09:36, 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, as well as a kernel commandline parameter to optionally
> disable BBML2 altogether.
> 
> This is a system feature as we might have a big.LITTLE architecture
> where some cores support BBML2 and some don't, but we want all cores to
> be available and BBM to default to level 0 (as opposed to having cores
> without BBML2 not coming online).
> 
> 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.
> Support detection of such CPUs.
> 
> Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@....com>

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