linux-kernel - Re: [PATCH v4 06/14] arm64: Hook up cmdline parameter to allow mismatched 32-bit EL0

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Date:   Tue, 1 Dec 2020 16:56:44 +0000
From:   Will Deacon <will@...nel.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 06/14] arm64: Hook up cmdline parameter to allow
 mismatched 32-bit EL0

On Fri, Nov 27, 2020 at 01:17:59PM +0000, Qais Yousef wrote:
> On 11/24/20 15:50, Will Deacon wrote:
> > Allow systems with mismatched 32-bit support at EL0 to run 32-bit
> > applications based on a new kernel parameter.
> > 
> > Signed-off-by: Will Deacon <will@...nel.org>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 7 +++++++
> >  arch/arm64/kernel/cpufeature.c                  | 7 +++++++
> >  2 files changed, 14 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 526d65d8573a..f20188c44d83 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -289,6 +289,13 @@
> >  			do not want to use tracing_snapshot_alloc() as it needs
> >  			to be done where GFP_KERNEL allocations are allowed.
> >  
> > +	allow_mismatched_32bit_el0 [ARM64]
> > +			Allow execve() of 32-bit applications and setting of the
> > +			PER_LINUX32 personality on systems where only a strict
> > +			subset of the CPUs support 32-bit EL0. When this
> > +			parameter is present, the set of CPUs supporting 32-bit
> > +			EL0 is indicated by /sys/devices/system/cpu/aarch32_el0.
> 
> Shouldn't we document that a randomly selected 32-bit CPU will be prevented
> from being hotplugged out all the time to act as the last man standing for any
> currently running 32-bit application.
> 
> That was a mouthful! I'm sure you can phrase it better :-)

Sure, I'll have a go at adding something. Thanks.

> If we make this the last patch as it was before adding affinity handling, we
> can drop patch 4 more easily I think?

I'd still prefer to keep patch 4, but I can certainly try to move this one
later in the series.

Will