linux-kernel - Re: [PATCH] x86/xsave: Robustify and merge macros

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Date:	Fri, 3 Apr 2015 22:42:17 +0200
From:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH] x86/xsave: Robustify and merge macros

On Fri, Apr 03, 2015 at 07:48:24PM +0200, Borislav Petkov wrote:
> On Fri, Apr 03, 2015 at 07:33:06PM +0200, Quentin Casasnovas wrote:
> > > Basically, the idea was:
> > > 
> > > .skip len(repl1) - len(orig), 0x90
> > > .skip len(repl2) - len(repl1), 0x90
> > > 
> > > BUT!, for some reason I changed it to what's there now and I can't
> > > remember why anymore.
> > 
> > I think it would not work in the case where repl1 is smaller or equal than
> > orig_insn (i.e. no padding in the first .skip) but orig_insn is strictly
> > smaller than repl2 (since we're never comparing repl2 with insn in this
> > new-old code).
> 
> orig_insn=4
> repl1=3
> repl2=5
> 
> .skip 0, 0x90
> .skip 2, 0x90
> 
> I think that still works, only the padding is larger than it needs to
> be. And it is so many bytes larger as len(abs(repl1 - orig_insn)) is.
> 
> In the example above, we'll get two bytes padding while only 1 suffices.
>

Right.

> > Anything wrong with the two different approaches I've suggested in my
> > original mail?
> 
> Right now, I want to have a minimal fix for obvious reasons. We can
> always improve stuff later when there's more time.
> 

If you're happy with the extra padding in such cases then your second
approach looks okay to me.  But IMO, even if taking the '.if' directive
approach is certainly bigger LOC-wise, it should be much easier to review
in a rush than some other .skip trickery.

It all depends on your definition of minimal change really, and whether
that extra padding is acceptable or not for you :)

Quentin
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