linux-kernel - Re: [PATCH 4/8] s390/entry: workaround llvm's IAS limitations

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Date:   Wed, 11 May 2022 10:30:05 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Jonas Paulsson <paulsson@...ux.vnet.ibm.com>,
        Ulrich Weigand <ulrich.weigand@...ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Alexander Egorenkov <egorenar@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Andreas Krebbel <krebbel@...ux.ibm.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 4/8] s390/entry: workaround llvm's IAS limitations

Hi Heiko,

On Wed, May 11, 2022 at 02:05:28PM +0200, Heiko Carstens wrote:
> llvm's integrated assembler cannot handle immediate values which are
> calculated with two local labels:
> 
> <instantiation>:3:13: error: invalid operand for instruction
>  clgfi %r14,.Lsie_done - .Lsie_gmap
> 
> Workaround this by adding clang specific code which reads the specific
> value from memory. Since this code is within the hot paths of the kernel
> and adds an additional memory reference, keep the original code, and add
> ifdef'ed code.
> 
> Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>
> ---
>  arch/s390/kernel/entry.S | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
> index e1664b45090f..ff7a75078e93 100644
> --- a/arch/s390/kernel/entry.S
> +++ b/arch/s390/kernel/entry.S
> @@ -171,8 +171,19 @@ _LPP_OFFSET	= __LC_LPP
>  	.macro OUTSIDE reg,start,end,outside_label
>  	larl	%r14,\start
>  	slgrk	%r14,\reg,%r14
> +#ifdef CONFIG_CC_IS_CLANG

I intend to put this series through my build and boot test matrix later
today but one fly by comment in the meantime. Should this be
CONFIG_AS_IS_LLVM if this is an integrated assembler limitation, rather
than a clang one?

> +	clgfrl	%r14,.Lrange_size\@
> +#else
>  	clgfi	%r14,\end - \start
> +#endif
>  	jhe	\outside_label
> +#ifdef CONFIG_CC_IS_CLANG
> +	.section .rodata, "a"
> +	.align 4
> +.Lrange_size\@:
> +	.long	\end - \start
> +	.previous
> +#endif
>  	.endm
>  
>  	.macro SIEEXIT
> -- 
> 2.32.0
> 
> 

Cheers,
Nathan