linux-kernel - Re: next/master bisection: baseline.login on rk3288-rock2-square

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Date:   Thu, 4 Feb 2021 11:55:12 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Marc Zyngier <maz@...nel.org>
Cc:     Guillaume Tucker <guillaume.tucker@...labora.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Nicolas Pitre <nico@...xnic.net>,
        "kernelci-results@...ups.io" <kernelci-results@...ups.io>
Subject: Re: next/master bisection: baseline.login on rk3288-rock2-square

(cc Marc)

On Thu, 4 Feb 2021 at 11:48, Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Thu, Feb 04, 2021 at 11:27:16AM +0100, Ard Biesheuvel wrote:
> > Hi Russell,
> >
> > If Guillaume is willing to do the experiment, and it fixes the issue,
> > it proves that rk3288 is relying on the flush before the MMU is
> > disabled, and so in that case, the fix is trivial, and we can just
> > apply it.
> >
> > If the experiment fails (which would mean rk3288 does not tolerate the
> > cache maintenance being performed after cache off), it is going to be
> > hairy, and so it will definitely take more time.
> >
> > So in the latter case (or if Guillaume does not get back to us), I
> > think reverting my queued fix is the only sane option. But in that
> > case, may I suggest that we queue the revert of the original by-VA
> > change for v5.12 so it gets lots of coverage in -next, and allows us
> > an opportunity to come up with a proper fix in the same timeframe, and
> > backport the revert and the subsequent fix as a pair? Otherwise, we'll
> > end up in the situation where v5.10.x until today has by-va, v5.10.x-y
> > has set/way, and v5.10y+ has by-va again. (I don't think we care about
> > anything before that, given that v5.4 predates any of this)
>
> I'm suggesting dropping your fix (9052/1) and reverting
> "ARM: decompressor: switch to by-VA cache maintenance for v7 cores"
> which gets us to a point where _both_ regressions are fixed.
>

I understand, but we don't know whether doing so might regress other
platforms that were added in the mean time.

> I'm of the opinion that the by-VA patch was incorrect when it was
> merged (it caused a regression), and it's only a performance
> improvement.

It is a correctness improvement, not a performance improvement.

Without that change, the 32-bit ARM kernel cannot boot bare metal on
platforms with a system cache such as 8040 or SynQuacer, and can only
boot under KVM on such systems because of the special handling of
set/way instructions by the host.

The performance issue related to set/way ops under KVM was already
fixed by describing data and unified caches as 1 set and 1 way when
running in 32-bit mode.


> Our attempts so far to fix it are just causing other
> regressions. So, I think it is reasonable to revert both back to a
> known good point which has worked over a decade. If doing so causes
> regressions (which I think is unlikely), then that would be unfortunate
> but alas is a price that's worth paying to get back to a known good
> point - since then we're not stacking regression fixes on top of other
> regression fixes.
>

This is exactly why I am proposing to queue the revert of the original
patch for v5.12, and only backport it to v5.10 and v5.11 once we are
sure it does not break anything else.