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Date: Thu, 7 Mar 2019 12:02:13 -0800
From: Andy Lutomirski <>
To: Borislav Petkov <>
Cc: "H. Peter Anvin" <>, Nadav Amit <>, 
	Rick Edgecombe <>, Andy Lutomirski <>, 
	Ingo Molnar <>, LKML <>, X86 ML <>, 
	Thomas Gleixner <>, Dave Hansen <>, 
	Peter Zijlstra <>, Damian Tometzki <>, 
	linux-integrity <>, 
	LSM List <>, 
	Andrew Morton <>, 
	Kernel Hardening <>, Linux-MM <>, 
	Will Deacon <>, Ard Biesheuvel <>, 
	Kristen Carlson Accardi <>, "Dock, Deneen T" <>, 
	Kees Cook <>, Dave Hansen <>, 
	Masami Hiramatsu <>
Subject: Re: [PATCH v2 10/20] x86: avoid W^X being broken during modules loading

On Thu, Mar 7, 2019 at 9:06 AM Borislav Petkov <> wrote:
> On Thu, Mar 07, 2019 at 08:53:34AM -0800, wrote:
> > If we *do*, what is the issue here? Although boot_cpu_has() isn't
> > slow (it should in general be possible to reduce to one testb
> > instruction followed by a conditional jump) it seems that "avoiding an
> > alternatives slot" *should* be a *very* weak reason, and seems to me
> > to look like papering over some other problem.
> Forget the current thread: this is simply trying to document when to use
> static_cpu_has() and when to use boot_cpu_has(). I get asked about it at
> least once a month.
> And then it is replacing clear slow paths using static_cpu_has() with
> boot_cpu_has() because there's purely no need to patch there. And having
> a RIP-relative MOV and a JMP is good enough for slow paths.

Should we maybe rename these functions?  static_cpu_has() is at least
reasonably obvious.  But cpu_feature_enabled() is different for
reasons I've never understood, and boot_cpu_has() is IMO terribly
named.  It's not about the boot cpu -- it's about doing the same thing
but with less bloat and less performance.

(And can we maybe collapse cpu_feature_enabled() and static_cpu_has()
into the same function?)


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