linux-kernel - Re: [PATCH v5 00/15] An alternative series for asymmetric AArch32 systems

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Message-ID: <20201217105533.GV3040@hirez.programming.kicks-ass.net>
Date:   Thu, 17 Dec 2020 11:55:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
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>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....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 v5 00/15] An alternative series for asymmetric AArch32
 systems

On Tue, Dec 15, 2020 at 06:50:12PM +0000, Will Deacon wrote:
> On Tue, Dec 15, 2020 at 06:36:45PM +0100, Peter Zijlstra wrote:

> > IOW, any (accidental or otherwise) trip through a 32bit helper, will
> > destroy user state (the affinity mask: 0x3c).
> 
> Yes, that's correct, and I agree that it's a rough edge. If you're happy
> with the idea of adding an extra mask to make this work, then I can start
> hacking that up

Yeah, I'm afraid we'll have to, this asymmetric muck is only going to
get worse from here on.

Anyway, I think we can avoid adding another cpumask_t to task_struct and
do with a cpumask_t * insteads. After all, for 'normal' tasks, the
task_cpu_possible_mask() will be cpu_possible_mask and we don't need to
carry anything extra.

Only once we hit one of these assymetric ISA things, can the task
allocate the additional cpumask and retain the full mask.

> (although I doubt I'll get something out before the new
> year at this point).

Yeah, we're all about to shut down for a bit, I'll not be looking at
email for 2 weeks either, so even if you send it, I might not see it
until the next year.