linux-kernel - Re: arm64 kvm built with clang doesn't boot

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Date:   Fri, 16 Mar 2018 15:50:19 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, LKML <linux-kernel@...r.kernel.org>,
        kernel-dynamic-tools <kernel-dynamic-tools@...gle.com>
Subject: Re: arm64 kvm built with clang doesn't boot

On Fri, Mar 16, 2018 at 3:13 PM, Mark Rutland <mark.rutland@....com> wrote:
> I think that patch is our best bet currently, but to save ourselves pain
> in future it would be *really* nice if GCC and clang could provide an
> option line -fno-absolute-addressing that would implicitly disable any
> feature that would generate an absolute address as jump tables do.
>

Let me know if you want me to mail that patch again.

Perhaps Nick can comment on whether something like
-fno-absolute-addressing would be feasible in clang. Although even if
it gets implemented, it won't fix the already released clang versions.

> With v4.15 (and clang 5.0.0), I did not have to disable jump labels to
> get a kernel booting on a Juno platform, though I did have to pass
> -fno-jump-tables to the hyp code.
>
> Which kernel version and clang version are you using?

I've rechecked and I think I was wrong here. I disabled
COFNIG_JUMP_LABEL while trying to get the kernel booting before I
added the kvm flags. It seems it's not needed after all.

But just for the reference, I'm using 4.16-rc4 with a patch to fix
SMCCC issues that you mentioned. As for clang, I'm using LLVM revision
325711 (a couple of weeks old).