linux-kernel - Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to entry_SYSENTER

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Message-ID: <20160225081441.GA11615@gmail.com>
Date:	Thu, 25 Feb 2016 09:14:41 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86/entry/32: Add an ASM_CLAC to
 entry_SYSENTER_32


* Andy Lutomirski <luto@...capital.net> wrote:

> >> Strengthen the SMAP protection by addding the missing ASM_CLAC right at the 
> >> beginning.
> >
> > Hmmm... this potentially adds a *lot* of unnecessary cycles to this path.  
> > Could we reinstate the early uaccess?
> 
> I think that's more trouble than it's worth, and it'll undo a bunch of the 
> context tracking cleanups that deferring it made possible, especially since this 
> only matters in a configuration (32-bit SMAP) that no one uses. [1]

But but ... 'context tracking' is not really something that a regular distro 
kernel cares about much - it's a nohz-full special AFAICS.

So if the only reason to keep this overhead is to simplify context tracking then 
I'm pretty sure we want to burden context-tracking with that, not the common fast 
path.

Anyway, maybe we are 'lucky':

> *However*, I just realized that I have no idea why the 32-bit sysenter
> path is safe against NT being set.  I fixed it on compat, and now I'm
> confused as to the status on 32-bit.  If we need to fix up NT, I think
> we can fold AC into that.
> 
> Also, I'll try to benchmark this soon.

Sounds good, thanks!

	Ingo