linux-kernel - Re: linux-next: build failure after merge of the tip tree

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Message-ID: <20140120091600.GW31570@twins.programming.kicks-ass.net>
Date:	Mon, 20 Jan 2014 10:16:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Len Brown <lenb@...nel.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, linux-next@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: build failure after merge of the tip tree

On Mon, Jan 20, 2014 at 12:56:47AM -0800, H. Peter Anvin wrote:
> On 01/20/2014 12:26 AM, Peter Zijlstra wrote:
> > On Sun, Jan 19, 2014 at 11:45:43PM -0500, Len Brown wrote:
> >> +static void mwait_idle(void)
> >> +{
> >> +       mwait_idle_with_hints(0, 0);
> >> +}
> >> +
> >>
> >> The reason the patch above will crash Core2 machines is because
> >> core2 machines don't support mwait_idle_with_hints().
> >>
> >> The calling sequence for old and new MWAIT instructions is different.
> >> The former must be invoked with interrupts enabled,
> >> and the later can be invoked with interrupts disabled,
> >> which is a feature that Linux takes advantage of.
> > 
> > What old and new? They're the same byte sequence: 0x0f 0x01 0xc9
> > 
> > And your 'old' __sti_mwait(0,0) has the exact same arguments as
> > mwait_idle_with_hints(0,0).
> > 
> 
> The difference is the STI!

So do the local_irq_enable(); mwait_idle_with_hints(0,0); thing.

But that's entirely different from saying that core2 doesn't support
mwait_idle_with_hints because its a different instruction.
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