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

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Message-ID: <3fd89842-86cd-ee01-0c2c-5438dd2d5f2a@arm.com>
Date:   Fri, 18 May 2018 19:13:43 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>, christoffer.dall@....com,
        Takahiro Akashi <takahiro.akashi@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH] arm64: kvm: use -fno-jump-tables with clang

On 18/05/18 18:56, Nick Desaulniers wrote:
> + Andrey
> 
> On Fri, May 18, 2018 at 10:45 AM Marc Zyngier <marc.zyngier@....com> wrote:
>> On 18/05/18 18:40, Nick Desaulniers wrote:
>>> On Fri, May 18, 2018 at 10:30 AM Marc Zyngier <marc.zyngier@....com>
> wrote:
>>>> I'm going to ask the question I've asked before when this patch cropped
>>>> up (must be the 4th time now):
> 
> The previous threads for context:
> https://patchwork.kernel.org/patch/10060381/
> https://lkml.org/lkml/2018/3/16/434
> 
>>>> Is it guaranteed that this is the only case where LLVM/clang is going
> to
>>>> generate absolute addresses instead of using relative addressing?
>>>
>>> It seems like if there's requirements that only relative addressing be
>>> used, then the compiler should be told explicitly about this
> restriction,
>>> no?
> 
>> Certainly. What's the rune?
> 
> It seems like -fno-jump-tables covers all known issues and unblocks people
> from doing further work.  It sounds like you'd like some kind of stronger
> guarantee? Wont those cases still "crop up" as far as needing to annotate
> either the code, or build scripts?

What I'd really like is to apply that patch knowing that:

- you have checked that with a released version of the compiler, you
don't observe any absolute address in any of the objects that are going
to be executed at EL2 on a mainline kernel,

- you have successfully run guests with a mainline kernel,

- it works for a reasonable set of common kernel configurations
(defconfig and some of the most useful debug options),

- I can reproduce your findings with the same released compiler.

Is that the case? I don't think any of the above is completely outlandish.

	M.
-- 
Jazz is not dead. It just smells funny...