linux-kernel - Re: [RFC][PATCH 02/22] x86,mmx

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Date:   Fri, 5 Nov 2021 09:05:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, mark.rutland@....com,
        dvyukov@...gle.com, seanjc@...gle.com, pbonzini@...hat.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 02/22] x86,mmx_32: Remove .fixup usage

On Thu, Nov 04, 2021 at 01:22:20PM -0700, Josh Poimboeuf wrote:
> On Thu, Nov 04, 2021 at 05:47:31PM +0100, Peter Zijlstra wrote:
> > This code puts an exception table entry on the "PREFIX" instruction to
> > overwrite it with a jmp.d8 when it triggers an exception. Except of
> > course, our code is no longer writable, also SMP.
> > 
> > Replace it with ALTERNATIVE, the novel
> > 
> > XXX: arguably we should just delete this code
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/lib/mmx_32.c |   83 ++++++++++++++++----------------------------------
> >  1 file changed, 27 insertions(+), 56 deletions(-)
> > 
> > --- a/arch/x86/lib/mmx_32.c
> > +++ b/arch/x86/lib/mmx_32.c
> > @@ -50,23 +50,17 @@ void *_mmx_memcpy(void *to, const void *
> >  	kernel_fpu_begin_mask(KFPU_387);
> >  
> >  	__asm__ __volatile__ (
> > -		"1: prefetch (%0)\n"		/* This set is 28 bytes */
> > -		"   prefetch 64(%0)\n"
> > -		"   prefetch 128(%0)\n"
> > -		"   prefetch 192(%0)\n"
> > -		"   prefetch 256(%0)\n"
> > -		"2:  \n"
> > -		".section .fixup, \"ax\"\n"
> > -		"3: movw $0x1AEB, 1b\n"	/* jmp on 26 bytes */
> > -		"   jmp 2b\n"
> > -		".previous\n"
> > -			_ASM_EXTABLE(1b, 3b)
> > -			: : "r" (from));
> > +		ALTERNATIVE "",
> > +			    "prefetch (%0)\n"
> > +			    "prefetch 64(%0)\n"
> > +			    "prefetch 128(%0)\n"
> > +			    "prefetch 192(%0)\n"
> > +			    "prefetch 256(%0)\n", X86_FEATURE_3DNOW
> 
> I think this should instead be X86_FEATURE_3DNOWPREFETCH (which isn't
> 3DNow-specific and should really just be called X86_FEATURE_PREFETCH
> anyway)

Ack.

> > +		: : "r" (from));
> >  
> >  	for ( ; i > 5; i--) {
> >  		__asm__ __volatile__ (
> > -		"1:  prefetch 320(%0)\n"
> > -		"2:  movq (%0), %%mm0\n"
> > +		"  movq (%0), %%mm0\n"
> 
> Not sure why this prefetch was removed?  It can also be behind an
> alternative just like the first one.

Urgh, that was me just plain killing all the prefetch in there and
forgetting to put it back :/ Will fix.