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

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Message-ID: <20250706101342.069b5068@pumpkin>
Date: Sun, 6 Jul 2025 10:13:42 +0100
From: David Laight <david.laight.linux@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Andy Lutomirski
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Subject: Re: [PATCHv8 02/17] x86/asm: Introduce inline memcpy and memset

On Thu, 3 Jul 2025 10:13:44 -0700
Dave Hansen <dave.hansen@...el.com> 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?
> 

It doesn't even really matter if there is an out-of-line memset.
All you need to do is 'teach' objtool it isn't a problem.

Is this for the boot-time asm-alternatives?
In that case I wonder why a 'low' address is being used?
With LASS enabled using a low address on a life kernel would make it
harder for another cpu to leverage the writable code page, but
that isn't a requirement of LASS.

If it is being used for later instruction patching you need the
very careful instruction sequences and cpu synchronisation.
In that case I suspect you need to add conditional stac/clac
to the existing patching code (and teach objtool it is all ok).

	David