linux-kernel - Re: [PATCH v2 06/18] arm64: kvm: use -fno-jump-tables with clang

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Message-ID: <20171116162518.GB94341@samitolvanen.mtv.corp.google.com>
Date:   Thu, 16 Nov 2017 08:25:18 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Alex Matveev <alxmtvv@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH v2 06/18] arm64: kvm: use -fno-jump-tables with clang

On Thu, Nov 16, 2017 at 11:46:17AM +0000, Will Deacon wrote:
> Can you elaborate a bit more on exactly what you saw failing here,
> please?

Mark noticed that clang built kernels fail to boot when the kernel
starts at EL2:

  http://lkml.iu.edu/hypermail/linux/kernel/1711.0/02817.html

Turns out starting with LLVM r308050, the compiler decides to use
a jump table in __init_stage2_translation, and generates code that
branches into an EL1 virtual address.

> Whilst it's obviously broken to jump to EL1 from EL2 w/o VHE, the
> commit message is a bit cryptic.

I agree, I'll add a more descriptive commit message for v3.

Sami