linux-kernel - Re: [PATCH v2] x86/math64: handle #DE in mul_u64_u64_div

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Message-ID: <20250728195353.631affdd@pumpkin>
Date: Mon, 28 Jul 2025 19:53:53 +0100
From: David Laight <david.laight.linux@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "Li,Rongqing" <lirongqing@...du.com>, Steven Rostedt <rostedt@...dmis.org>,
 linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2] x86/math64: handle #DE in mul_u64_u64_div_u64()

On Sun, 27 Jul 2025 14:34:58 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

> Change mul_u64_u64_div_u64() to return ULONG_MAX if the result doesn't
> fit into u64 or div == 0. The former matches the generic implementation
> in lib/math/div64.c, the latter doesn't. Perhaps we will add a WARN()
> into the fixup_exception() paths later.
> 
> No need to use _ASM_EXTABLE_TYPE_REG(), we know that the target register
> is pt_regs->ax with offset == 0, so a simple EX_DATA_REG(0) should work
> just fine.
> 
> Reported-by: Li RongQing <lirongqing@...du.com>
> Link: https://lore.kernel.org/all/78a0d7bb20504c0884d474868eccd858@baidu.com/
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Reviewed-by: David Laight <david.laight.linux@...il.com>

> ---
>  arch/x86/include/asm/div64.h | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/div64.h b/arch/x86/include/asm/div64.h
> index 9931e4c7d73f..0bf2c6afe66e 100644
> --- a/arch/x86/include/asm/div64.h
> +++ b/arch/x86/include/asm/div64.h
> @@ -79,18 +79,21 @@ static inline u64 mul_u32_u32(u32 a, u32 b)
>  
>  #else
>  # include <asm-generic/div64.h>
> +# include <asm/asm.h>
>  
>  /*
> - * Will generate an #DE when the result doesn't fit u64, could fix with an
> - * __ex_table[] entry when it becomes an issue.
> + * Returns ULONG_MAX if the result doesn't fit u64 or div == 0.
>   */
>  static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
>  {
>  	u64 q;
>  
> -	asm ("mulq %2; divq %3" : "=a" (q)
> -				: "a" (a), "rm" (mul), "rm" (div)
> -				: "rdx");
> +	asm ("mulq %2; 1: divq %3; 2:\n"
> +		_ASM_EXTABLE_TYPE(1b, 2b,
> +			EX_TYPE_IMM_REG | EX_DATA_REG(0) | EX_DATA_IMM(-1))
> +		: "=a" (q)
> +		: "a" (a), "rm" (mul), "rm" (div)
> +		: "rdx");
>  
>  	return q;
>  }