From: Claudio Jeker <cjeker@diehard.n-r-g.com>
Subject: Re: amd64: prefer enhanced REP MOVSB/STOSB feature if available
To: tech@openbsd.org
Date: Mon, 22 Dec 2025 13:31:49 +0100

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?
> 
> [0] https://marc.info/?l=openbsd-tech&m=176631121132731&w=2

Not my area but I think one issue we have since the introduction of
__HAVE_UVM_PERCPU and struct uvm_pmr_cache is that the the system
no longer uses the pre-zeroed pages provided by the zerothread.

I think fixing that and giving the system a way to have a per-cpu
magazin of zeroed pages results in far less cycles wasted holding
critical locks.
 
> 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);
> 
> 

-- 
:wq Claudio