Re: [PATCH RESEND] x86/kexec: store segment registers directly to memory in crash_setup_regs()

["H. Peter Anvin" <hpa@zytor.com>]

On 2026-09-06 11:39, Uros Bizjak wrote:
> crash_setup_regs() currently moves segment registers (ss, cs, ds, es)
> into pt_regs via a general purpose register (%eax). Update the code to
> avoid the intermediate register and store segment registers directly
> to their destination fields. This reduces the generated code from:
> 

> diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
> index 5cfb27f26583..ad7ef567452b 100644
> --- a/arch/x86/include/asm/kexec.h
> +++ b/arch/x86/include/asm/kexec.h
> @@ -106,11 +106,11 @@ static inline void crash_setup_regs(struct pt_regs *newregs,
>  		asm volatile("mov %%r14,%0" : "=m"(newregs->r14));
>  		asm volatile("mov %%r15,%0" : "=m"(newregs->r15));
>  #endif
> -		asm volatile("mov %%ss,%k0" : "=a"(newregs->ss));
> -		asm volatile("mov %%cs,%k0" : "=a"(newregs->cs));
> +		asm volatile("mov %%ss,%0" : "=m"(newregs->ss));
> +		asm volatile("mov %%cs,%0" : "=m"(newregs->cs));
>  #ifdef CONFIG_X86_32
> -		asm volatile("mov %%ds,%k0" : "=a"(newregs->ds));
> -		asm volatile("mov %%es,%k0" : "=a"(newregs->es));
> +		asm volatile("mov %%ds,%0" : "=m"(newregs->ds));
> +		asm volatile("mov %%es,%0" : "=m"(newregs->es));
>  #endif
>  		asm volatile("pushf\n\t"
>  			     "pop %0" : "=m"(newregs->flags));

Why not use "=rm"? "=a" seems odd in the extreme.

	-hpa