linux-kernel - Re: [PATCH] ARM: Add a memory clobber to the fmrx instruction

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Date: Tue, 9 Apr 2024 09:46:41 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: zhuqiuer <zhuqiuer1@...wei.com>
Cc: linux@...linux.org.uk, ndesaulniers@...gle.com, morbo@...gle.com,
	justinstitt@...gle.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, ardb@...nel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] ARM: Add a memory clobber to the fmrx instruction

+ Ard

On Tue, Apr 09, 2024 at 07:38:44PM +0800, zhuqiuer wrote:
> The instruction fmrx is used throughout the kernel,
> where it is sometimes expected to be skipped
> by incrementing the program counter, such as in vfpmodule.c:vfp_init().
> Therefore, the instruction should not be reordered when it is not intended.
> Adding a barrier() instruction before and after this call cannot prevent
> reordering by the compiler, as the fmrx instruction is constrained
> by '=r', meaning it works on the general register but not on memory.
> To ensure the order of the instruction after compiling,
> adding a memory clobber is necessary.
> 
> Below is the code snippet disassembled from the method:
> vfpmodule.c:vfp_init(), compiled by LLVM.
> 
> Before the patching:
> xxxxx:   xxxxx    bl  c010c688 <register_undef_hook>
> xxxxx:   xxxxx    mov r0, r4
> xxxxx:   xxxxx    bl  c010c6e4 <unregister_undef_hook>
> ...
> xxxxx:   xxxxx    bl  c0791c8c <printk>
> xxxxx:   xxxxx    movw    r5, #23132  ; 0x5a5c
> xxxxx:   xxxxx    vmrs    r4, fpsid  <- this is the fmrx instruction
> 
> After the patching:
> xxxxx:   xxxxx    bl  c010c688 <register_undef_hook>
> xxxxx:   xxxxx    mov r0, r4
> xxxxx:   xxxxx    vmrs    r5, fpsid  <- this is the fmrx instruction
> xxxxx:   xxxxx    bl  c010c6e4 <unregister_undef_hook>
> 
> Signed-off-by: zhuqiuer <zhuqiuer1@...wei.com>
> ---
>  arch/arm/vfp/vfpinstr.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm/vfp/vfpinstr.h b/arch/arm/vfp/vfpinstr.h
> index 3c7938fd40aa..e70129e10b8e 100644
> --- a/arch/arm/vfp/vfpinstr.h
> +++ b/arch/arm/vfp/vfpinstr.h
> @@ -68,7 +68,7 @@
>  	u32 __v;			\
>  	asm(".fpu	vfpv2\n"	\
>  	    "vmrs	%0, " #_vfp_	\
> -	    : "=r" (__v) : : "cc");	\
> +	    : "=r" (__v) : : "memory", "cc");	\
>  	__v;				\
>   })
>  
> -- 
> 2.12.3
> 

This seems like the same issue that Ard was addressing with this patch
at https://lore.kernel.org/20240318093004.117153-2-ardb+git@google.com/,
does that change work for your situation as well? I do not really have a
strong preference between the two approaches, Ard also mentioned using
*current in the asm constraints as another option.

Cheers,
Nathan