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

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Message-ID: <CAMj1kXHf0dNvsrfct6rCxi_yHXcQCqjwJoMa_TD0Fh6xo2zeZQ@mail.gmail.com>
Date:   Thu, 4 Feb 2021 15:25:20 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Marc Zyngier <maz@...nel.org>,
        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
Subject: Re: next/master bisection: baseline.login on rk3288-rock2-square

On Thu, 4 Feb 2021 at 15:09, Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Thu, Feb 04, 2021 at 12:26:44PM +0000, Marc Zyngier wrote:
> > I agree. With set/way CMOs, there is no way to reach the PoC if
> > it beyond the system cache, leading to an unbootable kernel.
> > This is actually pretty well documented in the architecture,
> > and it did bite us for the first time on XGene-1, 7 years ago.
>
> That may be, however we still do set/way maintenance to invalidate
> the L1 cache as that is required for ARMv7 to place the cache into
> a known state, as stated by the architecture reference manual.
>

Getting a certain cache at a certain level into a known state is a
valid use of set/way ops, and is simply unnecessary when running under
virtualization, but doesn't do any harm.

Pushing contents of the cache hierarchy to main memory is *not* a
valid use of set/way ops, and so there is no point in pretending that
set/way ops will produce the same results as by-VA ops. Only the by-VA
ops give the architectural guarantees that we rely on for correctness.

> Arguably, that should be done by firmware, but when starting
> secondary CPUs, there are platforms out there which do not bring
> the L1 cache to a defined state. So we are pretty much stuck with
> doing set/way operations during CPU initialisation in the main
> kernel.
>

Indeed. And this is unfortunate, but not the end of the world.

> If ARMv8 decides that this is not supportable, then that's a matter
> for ARMv8 to address without impacting the requirements of ARMv7.
>

I'm not sure what you mean here. The v7 architecture is crystal clear
about the difference between set/way ops (managing a single cache),
and by-VA ops (managing the 'cachedness' state of a memory region).
The semantics are radically different, regardless of v7 vs v8 or
AArch32 vs AArch64.