linux-kernel - Re: [PATCH v5 03/16] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <87jzdqt4w6.fsf@ubik.fi.intel.com>
Date: Wed, 30 Oct 2024 09:40:41 +0200
From: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>, Dave Hansen <dave.hansen@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave
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 <paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, Xiongwei Song
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Subject: Re: [PATCH v5 03/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

Peter Zijlstra <peterz@...radead.org> writes:

> On Tue, Oct 29, 2024 at 12:36:11PM +0100, Peter Zijlstra wrote:
>  static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
>  {
>  	void *ret = to;
>  
> -	asm volatile("rep movsb"
> -		     : "+D" (to), "+S" (from), "+c" (len)
> -		     : : "memory");
> -	return ret;
> +	asm volatile("1:\n\t"
> +		     ALT_64("rep movsb",
> +			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))

I don't know if it matters, but this basically brings in a whole memcpy
to a text_poke situation, which should only be a handful of bytes, and
creates a new stack frame in the !FSRM case, which the __always_inline
was intending to avoid. But given what text_poke is, maybe micro
optimizations don't really matter. And fewer memcpy() implementations
seems like a good idea.

Thanks,
--
Alex