linux-kernel - Re: [PATCH] x86/entry: add compatibility with IAS

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Date:   Sun, 12 Jul 2020 22:54:29 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Jian Cai <caij2003@...il.com>
Cc:     jiancai@...gle.com, ndesaulniers@...gle.com, manojgupta@...gle.com,
        sedat.dilek@...il.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86/entry: add compatibility with IAS

On Sun, Jul 12, 2020 at 06:24:22PM -0700, Jian Cai wrote:
> Clang's integrated assembler does not allow symbols with non-absolute
> values to be reassigned. This patch allows the affected code to be
> compatible with IAS.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1043
> Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Jian Cai <caij2003@...il.com>
> ---
>  arch/x86/include/asm/idtentry.h | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
> index f3d70830bf2a..77beed2cd6d9 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -468,34 +468,32 @@ __visible noinstr void func(struct pt_regs *regs,			\
>   */
>  	.align 8
>  SYM_CODE_START(irq_entries_start)
> -    vector=FIRST_EXTERNAL_VECTOR
> -    pos = .
> +    i = 1
> +    pos1 = .
>      .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
>  	UNWIND_HINT_IRET_REGS
> -	.byte	0x6a, vector
> +	.byte	0x6a, FIRST_EXTERNAL_VECTOR + i - 1
>  	jmp	asm_common_interrupt
>  	nop
>  	/* Ensure that the above is 8 bytes max */
> -	. = pos + 8
> -    pos=pos+8
> -    vector=vector+1
> +	. = pos1 + 8 * i
> +	i = i + 1
>      .endr
>  SYM_CODE_END(irq_entries_start)

I think it would be a little cleaner to initialize i to 0, and drop pos.
i.e. couldn't we do
	i = 0
	...
	.byte	0x6a, FIRST_EXTERNAL_VECTOR + i
	...
	i = i + 1
	. = irq_entries_start + 8 * i

>  
>  #ifdef CONFIG_X86_LOCAL_APIC
>  	.align 8
>  SYM_CODE_START(spurious_entries_start)
> -    vector=FIRST_SYSTEM_VECTOR
> -    pos = .
> +    i = 1
> +    pos2 = .
>      .rept (NR_VECTORS - FIRST_SYSTEM_VECTOR)
>  	UNWIND_HINT_IRET_REGS
> -	.byte	0x6a, vector
> +	.byte	0x6a, FIRST_SYSTEM_VECTOR + i - 1
>  	jmp	asm_spurious_interrupt
>  	nop
>  	/* Ensure that the above is 8 bytes max */
> -	. = pos + 8
> -    pos=pos+8
> -    vector=vector+1
> +	. = pos2 + 8 * i
> +	i = i + 1
>      .endr
>  SYM_CODE_END(spurious_entries_start)
>  #endif
> -- 
> 2.27.0.383.g050319c2ae-goog
>