linux-kernel - Re: Clang build of arm64 kernel fails

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Date:   Thu, 1 Mar 2018 13:31:16 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Marc Zyngier <marc.zyngier@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, ard.biesheuvel@...aro.org,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Clang build of arm64 kernel fails

On Thu, Mar 1, 2018 at 11:47 AM, Marc Zyngier <marc.zyngier@....com> wrote:
> Hi Andrey,
>
> On 28/02/18 19:32, Andrey Konovalov wrote:
>> Hi Marc!
>>
>> I've tried to pull in new upstream commits and the kernel build
>> started failing for me with the following errors (see below).
>>
>> It seems that the reason is your commit "arm64: Add
>> ARM_SMCCC_ARCH_WORKAROUND_1 BP hardening support". It seems that Clang
>> doesn't like 32 bits registers being used in 64 bits build.
>
> These are not AArch32 registers. They are AArch64 registers that the
> compiler specialises to an x (64bit wide) or w (32bit wide) register
> depending on the type that is used to define the corresponding variable.
> The fact that they are named just as AArch32 is an added compatibility
> bonus.
>
> For example:
>
> void foo(void)
> {
>         register unsigned long reg0 asm("r0") = 0;
>         register unsigned int  reg1 asm("r1") = 1;
>
>         asm volatile("hvc #0" : : "r" (reg0), "r" (reg1));
> }
>
> results in:
>
> Disassembly of section .text:
>
> 0000000000000000 <foo>:
>    0:   d2800000        mov     x0, #0x0                        // #0
>    4:   52800021        mov     w1, #0x1                        // #1
>    8:   d4000002        hvc     #0x0
>    c:   d65f03c0        ret

OK, I see, thanks for the explanation! I assumed there was something
wrong with the patch, since before it was applied everything was
compiling fine.

>
>> Would you mind sending a fix?
>
> I can look into it. Is there an equivalent Clang construct that wouldn't
> result in a sea of #ifdefs?

I'm having trouble to find any LLVM documentation related to this.

Kostya and Evgeniy, whom from the LLVM side we can ask about this?

>
> Thanks,
>
>         M.
> --
> Jazz is not dead. It just smells funny...