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

David Laight
2025-07-06T09:14:04Z

On Thu, 3 Jul 2025 10:13:44 -0700
Dave Hansen <dave.hansen@intel.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