linux-kernel - Re: [PATCH] x86/alternative: Optimize single-byte NOPs at an arbitrary position

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Message-ID: <YLdfbJBk20HIjfDE@zn.tnic>
Date:   Wed, 2 Jun 2021 12:37:32 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Richard Narron <richard@...zen.com>
Subject: Re: [PATCH] x86/alternative: Optimize single-byte NOPs at an
 arbitrary position

On Wed, Jun 02, 2021 at 10:14:58AM +0200, Peter Zijlstra wrote:
> Who again insisted that wouldn't happen? :-)

Yours truly, ofc. :-P

> That's almost a proper comment, might as well finish it
> 
> /*
>  * optimize_nops_range() - Optimize a sequence of single byte NOPs (0x90)
>  * @instr: instruction byte stream
>  * @instrlen: length of the above
>  * @off: offset within @instr where the first NOP has been detected
>  *
>  * Return: number of NOPs found (and replaced)
>  */

Done.

> 	for (; i < instrlen && instr[i] == 0x90; i++)
> 		;
> 
> perhaps? (possibly too dense, up to you)

Yeah, let's leave it simple so that one can read it at a quick glance.

> > +
> > +	nnops = i - off;
> > +
> > +	if (nnops <= 1)
> > +		return nnops;
> 
> !nnops would be an error, no?

Yeah, just being overly cautious. It should not be 0 since we're being
called for the byte at offset being 0x90. But I have said "should not
be" before and have fallen flat on my face.

> We really needs that extra line?

Zapped.

> Anyway, irrespective of nits:
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thanks, all except one incorporated.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette