Re: [PATCH v4 1/3] arm64: Add BBM Level 2 cpu feature

["Mikołaj Lenczewski" <miko.lenczewski@arm.com>]

On Thu, Mar 20, 2025 at 05:11:25PM +0000, Suzuki K Poulose wrote:
> On 20/03/2025 17:09, Mikołaj Lenczewski wrote:
> > On Thu, Mar 20, 2025 at 01:24:25PM +0000, Suzuki K Poulose wrote:
> > > On 19/03/2025 15:05, Mikołaj Lenczewski wrote:
> > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > > > index fb8752b42ec8..3e4cc917a07e 100644
> > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > @@ -453,6 +453,9 @@
> > > >    	arm64.no32bit_el0 [ARM64] Unconditionally disable the execution of
> > > >    			32 bit applications.
> > > > +	arm64.nobbml2	[ARM64] Unconditionally disable Break-Before-Make Level
> > > > +			2 support
> > > > +
> > > >    	arm64.nobti	[ARM64] Unconditionally disable Branch Target
> > > >    			Identification support
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index 940343beb3d4..49deda2b22ae 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -2057,6 +2057,17 @@ config ARM64_TLB_RANGE
> > > >    	  The feature introduces new assembly instructions, and they were
> > > >    	  support when binutils >= 2.30.
> > > > +config ARM64_BBML2_NOABORT
> > > > +	bool "Enable support for Break-Before-Make Level 2 detection and usage"
> > > > +	default y
> > > > +	help
> > > > +	  FEAT_BBM provides detection of support levels for break-before-make
> > > > +	  sequences. If BBM level 2 is supported, some TLB maintenance requirements
> > > > +	  can be relaxed to improve performance. We additonally require the
> > > > +	  property that the implementation cannot ever raise TLB Conflict Aborts.
> > > > +	  Selecting N causes the kernel to fallback to BBM level 0 behaviour
> > > > +	  even if the system supports BBM level 2.
> > > 
> > > minor nit: Should we mention that the feature can be disabled at runtime
> > > using a kernel parameter ?
> > 
> > Yes, this sounds very reasonable, I should have thought of that. Will
> > mention the commandline parameter in the kconfig option documentation.
> 
> And also may be mention this in the patch description and may be also
> add the rationale for providing this tunable.
> 
> Cheers
> Suzuki
> 

Will do! :)

-- 
Kind regards,
Mikołaj Lenczewski