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

[Kirill A. Shutemov]

On Thu, Jul 03, 2025 at 09:44:17AM +0100, David Laight wrote:
> On Tue,  1 Jul 2025 12:58:31 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> 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.
> 
> Except they contain the fault handlers so aren't generic calls.

That's true. I will add a comment to clarify it.

> > Originally-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/all/20241029184840.GJ14555@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/string.h     | 46 +++++++++++++++++++++++++++++++
> >  arch/x86/include/asm/uaccess_64.h | 38 +++++++------------------
> >  arch/x86/lib/clear_page_64.S      | 13 +++++++--
> >  3 files changed, 67 insertions(+), 30 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
> > index c3c2c1914d65..17f6b5bfa8c1 100644
> > --- a/arch/x86/include/asm/string.h
> > +++ b/arch/x86/include/asm/string.h
> > @@ -1,6 +1,52 @@
> >  /* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_X86_STRING_H
> > +#define _ASM_X86_STRING_H
> > +
> > +#include <asm/asm.h>
> > +#include <asm/alternative.h>
> > +#include <asm/cpufeatures.h>
> > +
> >  #ifdef CONFIG_X86_32
> >  # include <asm/string_32.h>
> >  #else
> >  # include <asm/string_64.h>
> >  #endif
> > +
> > +#ifdef CONFIG_X86_64
> > +#define ALT_64(orig, alt, feat) ALTERNATIVE(orig, alt, feat)
> > +#else
> > +#define ALT_64(orig, alt, feat) orig "\n"
> > +#endif
> > +
> > +static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
> > +{
> > +	void *ret = to;
> > +
> > +	asm volatile("1:\n\t"
> > +		     ALT_64("rep movsb",
> > +			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
> > +		     "2:\n\t"
> > +		     _ASM_EXTABLE_UA(1b, 2b)
> > +		     : "+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
> > +		     : : "memory", _ASM_AX);
> > +
> > +	return ret + len;
> > +}
> > +
> > +static __always_inline void *__inline_memset(void *addr, int v, size_t len)
> > +{
> > +	void *ret = addr;
> > +
> > +	asm volatile("1:\n\t"
> > +		     ALT_64("rep stosb",
> > +			    "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRM))
> > +		     "2:\n\t"
> > +		     _ASM_EXTABLE_UA(1b, 2b)
> > +		     : "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT
> > +		     : "a" ((uint8_t)v)
> 
> You shouldn't need the (uint8_t) cast (should that be (u8) anyway).
> At best it doesn't matter, at worst it will add code to mask with 0xff.

Right, will drop.

> > +		     : "memory", _ASM_SI, _ASM_DX);
> > +
> > +	return ret + len;
> > +}
> > +
> > +#endif /* _ASM_X86_STRING_H */
> > diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
> > index c8a5ae35c871..eb531e13e659 100644
> > --- a/arch/x86/include/asm/uaccess_64.h
> > +++ b/arch/x86/include/asm/uaccess_64.h
> > @@ -13,6 +13,7 @@
> >  #include <asm/page.h>
> >  #include <asm/percpu.h>
> >  #include <asm/runtime-const.h>
> > +#include <asm/string.h>
> >  
> >  /*
> >   * Virtual variable: there's no actual backing store for this,
> > @@ -118,21 +119,12 @@ rep_movs_alternative(void *to, const void *from, unsigned len);
> >  static __always_inline __must_check unsigned long
> >  copy_user_generic(void *to, const void *from, unsigned long len)
> >  {
> > +	void *ret;
> > +
> >  	stac();
> > -	/*
> > -	 * If CPU has FSRM feature, use 'rep movs'.
> > -	 * Otherwise, use rep_movs_alternative.
> > -	 */
> > -	asm volatile(
> > -		"1:\n\t"
> > -		ALTERNATIVE("rep movsb",
> > -			    "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
> > -		"2:\n"
> > -		_ASM_EXTABLE_UA(1b, 2b)
> > -		:"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
> > -		: : "memory", "rax");
> > +	ret = __inline_memcpy(to, from, len);
> >  	clac();
> > -	return len;
> > +	return ret - to;
> >  }
> >  
> >  static __always_inline __must_check unsigned long
> > @@ -178,25 +170,15 @@ rep_stos_alternative(void __user *addr, unsigned long len);
> >  
> >  static __always_inline __must_check unsigned long __clear_user(void __user *addr, unsigned long size)
> >  {
> > +	void *ptr = (__force void *)addr;
> > +	void *ret;
> > +
> >  	might_fault();
> >  	stac();
> > -
> > -	/*
> > -	 * No memory constraint because it doesn't change any memory gcc
> > -	 * knows about.
> > -	 */
> > -	asm volatile(
> > -		"1:\n\t"
> > -		ALTERNATIVE("rep stosb",
> > -			    "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS))
> > -		"2:\n"
> > -	       _ASM_EXTABLE_UA(1b, 2b)
> > -	       : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT
> > -	       : "a" (0));
> > -
> > +	ret = __inline_memset(ptr, 0, size);
> >  	clac();
> >  
> > -	return size;
> > +	return ret - ptr;
> >  }
> >  
> >  static __always_inline unsigned long clear_user(void __user *to, unsigned long n)
> > diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
> > index a508e4a8c66a..47b613690f84 100644
> > --- a/arch/x86/lib/clear_page_64.S
> > +++ b/arch/x86/lib/clear_page_64.S
> > @@ -55,17 +55,26 @@ SYM_FUNC_END(clear_page_erms)
> >  EXPORT_SYMBOL_GPL(clear_page_erms)
> >  
> >  /*
> > - * Default clear user-space.
> > + * Default memset.
> >   * Input:
> >   * rdi destination
> > + * rsi scratch
> >   * rcx count
> > - * rax is zero
> > + * al is value
> >   *
> >   * Output:
> >   * rcx: uncleared bytes or 0 if successful.
> > + * rdx: clobbered
> >   */
> >  SYM_FUNC_START(rep_stos_alternative)
> >  	ANNOTATE_NOENDBR
> > +
> > +	movzbq %al, %rsi
> > +	movabs $0x0101010101010101, %rax
> > +
> > +	/* RDX:RAX = RAX * RSI */
> > +	mulq %rsi
> 
> NAK - you can't do that here.
> Neither %rsi nor %rdx can be trashed.
> The function has a very explicit calling convention.

What calling convention? We change the only caller to confirm to this.

> It is also almost certainly a waste of time.
> Pretty much all the calls will be for a constant 0x00.
> Rename it all memzero() ...

text_poke_memset() is not limited to zeroing.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov