linux-kernel - Re: Sysenter crash with Nested Task Bit set

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Date:	Mon, 18 Sep 2006 18:39:45 +0200
From:	Andi Kleen <ak@...e.de>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Linus Torvalds <torvalds@...l.org>, Andrew Morton <akpm@...l.org>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	In Cognito <defend.the.world@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Sysenter crash with Nested Task Bit set

On Monday 18 September 2006 18:12, Benjamin LaHaise wrote:
> On Mon, Sep 18, 2006 at 05:29:23PM +0200, Andi Kleen wrote:
> > > -	asm volatile("pushl %%ebp\n\t"					\
> > > +	asm volatile("pushfl\n\t"		/* Save flags */	\
> > > +		     "pushl %%ebp\n\t"					\
> > 
> > We used to do that pushfl/popfl some time ago, but Ben removed it because
> > it was slow on P4.  Ok, nobody thought of that case back then.
> 
> It's the pushfl that will be slow on any OoO CPU, as it has dependancies on  
> any previous instructions that modified the flags, which ends up bringing 
> all of the memory ordering dependancies into play.  Doing a popfl to set the 
> flags to some known value is much less expensive.

Yes it's never fast, but on basically all non P4 CPUs it is still fast enough
to not be a problem. I suppose it causes a trace cache flush or something like
that there.

-Andi
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