RE: [PATCH] x86/asm/64: Align start of __clear_user() loop to 16-bytes

[David Laight <David.Laight@ACULAB.COM>]

From: Alexey Dobriyan 
> Sent: 18 June 2020 14:17
...
> > > diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
> > > index fff28c6f73a2..b0dfac3d3df7 100644
> > > --- a/arch/x86/lib/usercopy_64.c
> > > +++ b/arch/x86/lib/usercopy_64.c
> > > @@ -24,6 +24,7 @@ unsigned long __clear_user(void __user *addr, unsigned long size)
> > >  	asm volatile(
> > >  		"	testq  %[size8],%[size8]\n"
> > >  		"	jz     4f\n"
> > > +		"	.align 16\n"
> > >  		"0:	movq $0,(%[dst])\n"
> > >  		"	addq   $8,%[dst]\n"
> > >  		"	decl %%ecx ; jnz   0b\n"
> >
> > You can do better that that loop.
> > Change 'dst' to point to the end of the buffer, negate the count
> > and divide by 8 and you get:
> > 		"0:	movq $0,($[dst],%%ecx,8)\n"
> > 		"	add $1,%%ecx"
> > 		"	jnz 0b\n"
> > which might run at one iteration per clock especially on cpu that pair
> > the add and jnz into a single uop.
> > (You need to use add not inc.)
> 
> /dev/zero should probably use REP STOSB etc just like everything else.

Almost certainly it shouldn't, and neither should anything else.
Potentially it could use whatever memset() is patched to.
That MIGHT be 'rep stos' on some cpu variants, but in general
it is slow.

	David

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