linux-kernel - Re: [PATCH v4 04/14] arm64: Kill 32-bit applications scheduled on 64-bit-only CPUs

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Date:   Wed, 2 Dec 2020 17:42:48 +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 04/14] arm64: Kill 32-bit applications scheduled on
 64-bit-only CPUs

On Wed, Dec 02, 2020 at 01:52:16PM +0000, Qais Yousef wrote:
> On 12/01/20 16:56, Will Deacon wrote:
> > On Fri, Nov 27, 2020 at 01:12:17PM +0000, Qais Yousef wrote:
> > > On 11/24/20 15:50, Will Deacon wrote:
> > > > Scheduling a 32-bit application on a 64-bit-only CPU is a bad idea.
> > > > 
> > > > Ensure that 32-bit applications always take the slow-path when returning
> > > > to userspace on a system with mismatched support at EL0, so that we can
> > > > avoid trying to run on a 64-bit-only CPU and force a SIGKILL instead.
> > > > 
> > > > Signed-off-by: Will Deacon <will@...nel.org>
> > > > ---
> > > 
> > > nit: We drop this patch at the end. Can't we avoid it altogether instead?
> > 
> > I did it like this so that the last patch can be reverted for
> > testing/debugging, but also because I think it helps the structure of the
> > series.
> 
> Cool. I had a comment about the barrier(), you were worried about
> cpu_affinity_invalid() being inlined by the compiler and then things get
> mangled such that TIF_NOTIFY_RESUME clearing is moved after the call as you
> described? Can the compiler move things if cpu_affinity_invalid() is a proper
> function call (not inlined)?

I think function calls implicitly clobber memory, but you'd have to annotate
the thing as noinline to prevent it being inlined.

Will