Re: mainline/master bisection: baseline.login on meson-sm1-khadas-vim3l

From: Marc Zyngier <maz@kernel.org>
To: Guillaume Tucker <guillaume.tucker@collabora.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	David Brazdil <dbrazdil@google.com>,
	"kernelci-results@groups.io" <kernelci-results@groups.io>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Remi Denis-Courmont <remi.denis.courmont@huawei.com>,
	linux-kernel@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	Andrey Konovalov <andreyknvl@google.com>,
	Amit Daniel Kachhap <amit.kachhap@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: mainline/master bisection: baseline.login on meson-sm1-khadas-vim3l
Date: Wed, 24 Feb 2021 08:52:30 +0000	[thread overview]
Message-ID: <877dmx26gh.wl-maz@kernel.org> (raw)
In-Reply-To: <d9156098-f512-09bc-fa42-7a0f0bdee978@collabora.com>

On Tue, 23 Feb 2021 21:03:52 +0000,
Guillaume Tucker <guillaume.tucker@collabora.com> wrote:
> 
> On 23/02/2021 14:18, Marc Zyngier wrote:
> > Hi Guillaume,
> > 
> > On Tue, 23 Feb 2021 09:46:30 +0000,
> > Guillaume Tucker <guillaume.tucker@collabora.com> wrote:
> >>
> >> Hello Marc,
> >>
> >> Please see the bisection report below about a boot failure on
> >> meson-sm1-khadas-vim3l on mainline.  It seems to only be
> >> affecting kernels built with CONFIG_ARM64_64K_PAGES=y.
> >>
> >> Reports aren't automatically sent to the public while we're
> >> trialing new bisection features on kernelci.org but this one
> >> looks valid.
> >>
> >> There's no output in the log, so the kernel is most likely
> >> crashing early.  Some more details can be found here:
> >>
> >>   https://kernelci.org/test/case/id/6034bed3b344e2860daddcc8/
> >>
> >> Please let us know if you need any help to debug the issue or try
> >> a fix on this platform.
> > 
> > Thanks for the heads up.
> > 
> > There is actually a fundamental problem with the patch you bisected
> > to: it provides no guarantee that the point where we enable the EL2
> > MMU is in the idmap and, as it turns out, the code we're running from
> > disappears from under our feet, leading to a translation fault we're
> > not prepared to handle.
> > 
> > How does it work with 4kB pages? Luck.
> 
> There may be a fascinating explanation for it, but luck works
> too.  It really seems to be booting happily with 4k pages:
> 
>   https://kernelci.org/test/plan/id/60347b358de339d1b7addcc5/

Oh, I know it boots fine with 4k, that's what I used everywhere.
We're just lucky that the bit of code that deals with the MMU happens
to *also* be in the idmap. With 64k pages, it gets pushed further down
the line, and bad things happen. Short of explicit statements in the
code, luck rules.

> 
> > Do you mind giving the patch below a go? It does work on my vim3l and
> > on a FVP, so odds are that it will solve it for you too.
> 
> Sure, and that worked here as well:
> 
>   http://lava.baylibre.com:10080/scheduler/job/752416
> 
> and here's the test branch where I applied your fix, for
> completeness:
> 
>   https://gitlab.collabora.com/gtucker/linux/-/commits/v5.11-vim3l-vhe/

Awesome. thanks for having tested it.

> As always, if you do send a patch with the fix, please give some
> credit to the bot:
> 
>   Reported-by: "kernelci.org bot" <bot@kernelci.org> 

Will do. Mind if I credit you too for the testing?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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