From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v4 24/26] arm64: mte: Introduce early param to disable
 MTE support
Date: Tue, 19 May 2020 17:14:34 +0100
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To: Vladimir Murzin <vladimir.murzin@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, Szabolcs Nagy <szabolcs.nagy@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Kevin Brodsky <kevin.brodsky@arm.com>, Peter Collingbourne <pcc@google.com>, linux-mm@kvack.org, Vincenzo Frascino <vincenzo.frascino@arm.com>, Will Deacon <will@kernel.org>, Dave P Martin <Dave.Martin@arm.com>

On Mon, May 18, 2020 at 12:26:30PM +0100, Vladimir Murzin wrote:
> On 5/15/20 6:16 PM, Catalin Marinas wrote:
> > For performance analysis it may be desirable to disable MTE altogether
> > via an early param. Introduce arm64.mte_disable and, if true, filter out
> > the sanitised ID_AA64PFR1_EL1.MTE field to avoid exposing the HWCAP to
> > user.
> > 
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > ---
> > 
> > Notes:
> >     New in v4.
> > 
> >  Documentation/admin-guide/kernel-parameters.txt |  4 ++++
> >  arch/arm64/kernel/cpufeature.c                  | 11 +++++++++++
> >  2 files changed, 15 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index f2a93c8679e8..7436e7462b85 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -373,6 +373,10 @@
> >  	arcrimi=	[HW,NET] ARCnet - "RIM I" (entirely mem-mapped) cards
> >  			Format: <io>,<irq>,<nodeID>
> >  
> > +	arm64.mte_disable=
> > +			[ARM64] Disable Linux support for the Memory
> > +			Tagging Extension (both user and in-kernel).
> > +
> 
> Should it really to take parameter (on/off/true/false)? It may lead to expectation
> that arm64.mte_disable=false should enable MT and, yes, double negatives make it
> look ugly, so if we do need parameter, can it be arm64.mte=on/off/true/false?

My reasoning about arm64.mte= was that 'on' may lead people to think it
does something even when MTE isn't available on the SoC. So I ended up
with an explicit 'disable' in the name. Happy to change it if we don't
drop this parameter altogether (in the absence of valid use-cases).

-- 
Catalin