linux-kernel - Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

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Message-ID: <20250313181319.GB40525@mazurka.cambridge.arm.com>
Date: Thu, 13 Mar 2025 18:13:19 +0000
From: Mikołaj Lenczewski <miko.lenczewski@....com>
To: Yang Shi <yang@...amperecomputing.com>, ryan.roberts@....com
Cc: suzuki.poulose@....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,
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	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev
Subject: Re: [PATCH v3 1/3] arm64: Add BBM Level 2 cpu feature

On Thu, Mar 13, 2025 at 10:21:51AM -0700, Yang Shi wrote:
> On 3/13/25 3:41 AM, 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
> 
> Hi Miko,
> 
> A question about the kernel boot parameter. Can this parameter be used 
> in early boot stage? A quick look at the code shows it should be ok, for 
> example, cpu_has_bti() is called in map_kernel(). But I'd like to double 
> check because my patchset needs to check this parameter in map_mem() to 
> determine whether large block mapping can be used or not.
> 
> And a nit below.

I will need to double check exactly when the arm64 software overrides
are finalised, but as long as those values are finalised in / before (?)
the early boot stage then it should be fine? Will reply again once I
check and have an answer.
   
> > +static inline bool bbml2_possible(void)
> > +{
> > +	return !arm64_test_sw_feature_override(ARM64_SW_FEATURE_OVERRIDE_NOBBML2);
> > +}
> 
> Can this be moved to cpufeature.h? My patch will use this, anyway I can 
> do it in my patchset.
> 
> Thanks,
> Yang

I can do so. In fact, on second thought, I will probably extend this to
also include the `IS_ENABLED(CONFIG_ARM64_BBML2_NOABORT)` check as well,
and then move it to cpufeature.h, instead of folding said check into
has_bbml2_noabort().

-- 
Kind regards,
Mikołaj Lenczewski