linux-kernel - Re: [PATCH v3 1/4] x86/clear_page: extend clear

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Message-ID: <20250414110259.GF5600@noisy.programming.kicks-ass.net>
Date: Mon, 14 Apr 2025 13:02:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, x86@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, bp@...en8.de,
	dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
	luto@...nel.org, paulmck@...nel.org, rostedt@...dmis.org,
	tglx@...utronix.de, willy@...radead.org, jon.grimm@....com,
	bharata@....com, raghavendra.kt@....com, boris.ostrovsky@...cle.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH v3 1/4] x86/clear_page: extend clear_page*() for
 multi-page clearing

On Mon, Apr 14, 2025 at 08:32:29AM +0200, Ingo Molnar wrote:

> >  static inline void clear_page(void *page)
> >  {
> > +	unsigned int length = PAGE_SIZE;
> >  	/*
> > -	 * Clean up KMSAN metadata for the page being cleared. The assembly call
> > +	 * Clean up KMSAN metadata for the pages being cleared. The assembly call
> >  	 * below clobbers @page, so we perform unpoisoning before it.
> 
> >  	 */
> > -	kmsan_unpoison_memory(page, PAGE_SIZE);
> > -	alternative_call_2(clear_page_orig,
> > -			   clear_page_rep, X86_FEATURE_REP_GOOD,
> > -			   clear_page_erms, X86_FEATURE_ERMS,
> > +	kmsan_unpoison_memory(page, length);
> > +
> > +	alternative_call_2(clear_pages_orig,
> > +			   clear_pages_rep, X86_FEATURE_REP_GOOD,
> > +			   clear_pages_erms, X86_FEATURE_ERMS,
> >  			   "=D" (page),
> > -			   "D" (page),
> > +			   ASM_INPUT("D" (page), "S" (length)),
> >  			   "cc", "memory", "rax", "rcx");
> >  }
> >  
> > diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
> > index a508e4a8c66a..bce516263b69 100644
> > --- a/arch/x86/lib/clear_page_64.S
> > +++ b/arch/x86/lib/clear_page_64.S
> > @@ -13,20 +13,35 @@
> >   */
> >  
> >  /*
> > - * Zero a page.
> > - * %rdi	- page
> > + * Zero kernel page aligned region.
> > + *
> > + * Input:
> > + * %rdi	- destination
> > + * %esi	- length
> > + *
> > + * Clobbers: %rax, %rcx
> >   */
> > -SYM_TYPED_FUNC_START(clear_page_rep)
> > -	movl $4096/8,%ecx
> > +SYM_TYPED_FUNC_START(clear_pages_rep)
> > +	movl %esi, %ecx
> >  	xorl %eax,%eax
> > +	shrl $3,%ecx
> >  	rep stosq
> >  	RET
> > -SYM_FUNC_END(clear_page_rep)
> > -EXPORT_SYMBOL_GPL(clear_page_rep)
> > +SYM_FUNC_END(clear_pages_rep)
> > +EXPORT_SYMBOL_GPL(clear_pages_rep)
> >  
> > -SYM_TYPED_FUNC_START(clear_page_orig)
> > +/*
> > + * Original page zeroing loop.
> > + * Input:
> > + * %rdi	- destination
> > + * %esi	- length
> > + *
> > + * Clobbers: %rax, %rcx, %rflags
> > + */
> > +SYM_TYPED_FUNC_START(clear_pages_orig)
> > +	movl   %esi, %ecx
> >  	xorl   %eax,%eax
> > -	movl   $4096/64,%ecx
> > +	shrl   $6,%ecx
> 
> So if the natural input parameter is RCX, why is this function using 
> RSI as the input 'length' parameter? Causes unnecessary register 
> shuffling.

This symbol is written as a C function with C calling convention, even
though it is only meant to be called from that clear_page() alternative.

If we want to go change all this, then we should go do the same we do
for __clear_user() and write it thusly:

	asm volatile(ALTERNATIVE("rep stosb",
				 "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS)
				 : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT
				 : "a" (0))

And forget about all those clear_page_*() thingies.