On Mon, Dec 22, 2025 at 01:23:18PM +0100, Martin Pieuchot wrote:
> As Mateusz Guzik pointed out recently [0] we can greatly reduce the
> amount of CPU cycles spent zeroing pages by using 'rep stosb'.
> 
> Diff below does that, ok?

For anyone wondering if their amd64 hardware will benefit from this,
the ERMS feature flag is set on about the last 10 years of Intel cpus,
and (I think) on Zen 4 and later for AMD cpus.

ok jmatthew@

> 
> [0] https://marc.info/?l=openbsd-tech&m=176631121132731&w=2
> 
> Index: arch/amd64/amd64/locore.S
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/amd64/locore.S,v
> diff -u -p -r1.151 locore.S
> --- arch/amd64/amd64/locore.S	2 Aug 2025 07:33:28 -0000	1.151
> +++ arch/amd64/amd64/locore.S	22 Dec 2025 11:54:32 -0000
> @@ -1172,6 +1172,16 @@ ENTRY(pagezero)
>  	lfence
>  END(pagezero)
>  
> +ENTRY(pagezero_erms)
> +	RETGUARD_SETUP(pagezero_erms, r11)
> +	movq    $PAGE_SIZE,%rcx
> +	xorq    %rax,%rax
> +	rep stosb
> +	RETGUARD_CHECK(pagezero_erms, r11)
> +	ret
> +	lfence
> +END(pagezero_erms)
> +
>  /* void pku_xonly(void) */
>  ENTRY(pku_xonly)
>  	movq	pg_xo,%rax	/* have PKU support? */
> Index: arch/amd64/amd64/pmap.c
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/amd64/pmap.c,v
> diff -u -p -r1.182 pmap.c
> --- arch/amd64/amd64/pmap.c	15 Aug 2025 13:40:43 -0000	1.182
> +++ arch/amd64/amd64/pmap.c	22 Dec 2025 11:55:07 -0000
> @@ -1594,11 +1594,14 @@ pmap_extract(struct pmap *pmap, vaddr_t 
>  /*
>   * pmap_zero_page: zero a page
>   */
> -
>  void
>  pmap_zero_page(struct vm_page *pg)
>  {
> -	pagezero(pmap_map_direct(pg));
> +	/* Prefer enhanced REP MOVSB/STOSB feature if available. */
> +	if (ISSET(curcpu()->ci_feature_sefflags_ebx, SEFF0EBX_ERMS))
> +		pagezero_erms(pmap_map_direct(pg));
> +	else
> +		pagezero(pmap_map_direct(pg));
>  }
>  
>  /*
> Index: arch/amd64/include/pmap.h
> ===================================================================
> RCS file: /cvs/src/sys/arch/amd64/include/pmap.h,v
> diff -u -p -r1.94 pmap.h
> --- arch/amd64/include/pmap.h	7 Jul 2025 00:55:15 -0000	1.94
> +++ arch/amd64/include/pmap.h	22 Dec 2025 11:46:09 -0000
> @@ -403,6 +403,7 @@ void		pmap_write_protect(struct pmap *, 
>  paddr_t	pmap_prealloc_lowmem_ptps(paddr_t);
>  
>  void	pagezero(vaddr_t);
> +void	pagezero_erms(vaddr_t);
>  
>  void	pmap_convert(struct pmap *, int);
>  void	pmap_enter_special(vaddr_t, paddr_t, vm_prot_t);
> 
>