linux-kernel - Re: [PATCH v3 0/3] Initial BBML2 support for contpte

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Message-ID: <e8749c86-fe93-4123-aa2e-5677e7a2c695@arm.com>
Date: Thu, 13 Mar 2025 11:22:07 +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,
 mark.rutland@....com, joey.gouly@....com, maz@...nel.org,
 james.morse@....com, broonie@...nel.org, anshuman.khandual@....com,
 oliver.upton@...ux.dev, ioworker0@...il.com, baohua@...nel.org,
 david@...hat.com, jgg@...pe.ca, shameerali.kolothum.thodi@...wei.com,
 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 v3 0/3] Initial BBML2 support for contpte_convert()

Hi Miko,

On 13/03/2025 10:41, Mikołaj Lenczewski wrote:
> Hi All,
> 
> This patch series adds adding initial support for eliding
> break-before-make requirements on systems that support BBML2 and
> additionally guarantee to never raise a conflict abort.
> 
> This support elides a TLB invalidation in contpte_convert(). This
> leads to a 12% improvement when executing a microbenchmark designed
> to force the pathological path where contpte_convert() gets called.
> This represents an 80% reduction in the cost of calling
> contpte_convert().
> 
> This series is based on v6.14-rc4 (d082ecbc71e9).
> 
> Patch 1 implements an allow-list of cpus that support BBML2, but with
> the additional constraint of never causing TLB conflict aborts. We
> settled on this constraint because we will use the feature for kernel
> mappings in the future, for which we cannot handle conflict aborts
> safely.
> 
> Yang Shi has a series at [1] that aims to use BBML2 to enable splitting
> the linear map at runtime. This series partially overlaps with it to add
> the cpu feature. We believe this series is fully compatible with Yang's
> requirements and could go first.
> 

Nothing about the patch functionality, but :

Minor nit: Generally it is a good idea to add "What changed" from the
previous posting.  That gives the reviewers an idea of what to look for
in the new version. Something like:

Changes since V2:
  {Adding a link to the posting is an added bonus, as we can look up the 
discussions easily}
  - blah blah
  - ..


Cheers
Suzuki


> [1]:
>    https://lore.kernel.org/linux-arm-kernel/20250304222018.615808-1-yang@os.amperecomputing.com/
> 
> Mikołaj Lenczewski (3):
>    arm64: Add BBM Level 2 cpu feature
>    iommu/arm: Add BBM Level 2 smmu feature
>    arm64/mm: Elide tlbi in contpte_convert() under BBML2
> 
>   .../admin-guide/kernel-parameters.txt         |  3 +
>   arch/arm64/Kconfig                            | 11 +++
>   arch/arm64/include/asm/cpucaps.h              |  2 +
>   arch/arm64/include/asm/cpufeature.h           |  6 ++
>   arch/arm64/kernel/cpufeature.c                | 76 +++++++++++++++++++
>   arch/arm64/kernel/pi/idreg-override.c         |  2 +
>   arch/arm64/mm/contpte.c                       |  3 +-
>   arch/arm64/tools/cpucaps                      |  1 +
>   .../iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c   |  3 +
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   |  3 +
>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |  4 +
>   11 files changed, 113 insertions(+), 1 deletion(-)
>