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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Mon, 9 Nov 2020 20:10:29 +0100
Message-ID: <CAK8P3a2tM1Gzy7Y98tiYGoNcLye77je_UCtTUQYcP2UuRNRKwQ@mail.gmail.com>
Subject: Re: [PATCH 2/3] arm: introduce IRQ stacks
To:     Tony Lindgren <tony@atomide.com>
Cc:     Russell King - ARM Linux admin <linux@armlinux.org.uk>,
        v.narang@samsung.com, a.sahrawat@samsung.com,
        Marc Zyngier <maz@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Vincent Whitchurch <vincent.whitchurch@axis.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Nathan Huckleberry <nhuck@google.com>,
        Jian Cai <caij2003@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Maninder Singh <maninder1.s@samsung.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Mon, Nov 9, 2020 at 3:45 PM Tony Lindgren <tony@atomide.com> wrote:
> >
> > As discussed on IRC, I think it can still be done in one of these
> > ways, though admittedly none of them are perfect:
> >
> > a) add runtime patching for __my_cpu_offset() when
> >   CONFIG_SMP_ON_UP is set. This adds complexity but avoids the
> >   fallback for for SMP&&CPU_V6. It possibly also speeds up
> >   running on single-cpu systems if the TPIDRPRW access adds
> >   any measurable runtime overhead compared to patching it out.
>
> Out of these options a) sounds best to me.

Ok. Maninder, would you like to give implementing this a try?

> > b) If irq stacks are left as a compile-time option, that could be
> >   made conditional on "!(SMP&&CPU_V6)". Presumably very
> >   few people still run kernels built that way any more. The only
> >   supported platforms are i.MX3, OMAP2 and Realview-eb, all of
> >   which are fairly uncommon these days and would usually
> >   run v6-only non-SMP kernels.
>
> This has been working just fine for years though. In general,
> removing the conditional compile ifdefferey has made things quite
> a bit easier for us, so let's continue on that.
>
> > c) If we decide that we no longer care about that configuration
> >   at all, we could decide to just make SMP depend on !CPU_V6,
> >   and possibly kill off the entire SMP_ON_UP patching logic.
> >   I suspect we still want to keep SMP_ON_UP for performance
> >   reasons, but I don't know how significant they are to start with.
>
> And this too has been working just fine for years :)

I know it works, my point was that I'm not sure anyone cares
any more ;-)

I suppose the existence of omap2plus_defconfig and
imx_v6_v7_defconfig means it does at least get tested
regularly.

       Arnd