linux-kernel - Re: [PATCHv8 02/17] x86/asm: Introduce inline memcpy and memset

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Message-ID: <otaa7t5kq4wtj5zk3vis7ldqw56i7bwzga7yg7pskgeps7zthg@fthlgikkmlhh>
Date: Fri, 4 Jul 2025 12:04:25 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: 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>, 
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Subject: Re: [PATCHv8 02/17] x86/asm: Introduce inline memcpy and memset

On Thu, Jul 03, 2025 at 10:13:44AM -0700, Dave Hansen wrote:
> On 7/1/25 02:58, Kirill A. Shutemov wrote:
> > Extract memcpy and memset functions from copy_user_generic() and
> > __clear_user().
> > 
> > They can be used as inline memcpy and memset instead of the GCC builtins
> > whenever necessary. LASS requires them to handle text_poke.
> 
> Why are we messing with the normal user copy functions? Code reuse is
> great, but as you're discovering, the user copy code is highly
> specialized and not that easy to reuse for other things.
> 
> Don't we just need a dirt simple chunk of code that does (logically):
> 
> 	stac();
> 	asm("rep stosq...");
> 	clac();
> 
> Performance doesn't matter for text poking, right? It could be stosq or
> anything else that you can inline. It could be a for() loop for all I
> care as long as the compiler doesn't transform it into some out-of-line
> memset. Right?

Yeah, performance doesn't matter for text poking. And this approach
simplifies the code quite a bit. I will use direct asm() for text poke.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov