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

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Date:	Mon, 20 Jan 2014 01:23:00 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 01/20/2014 01:16 AM, Peter Zijlstra wrote:
>>
>> The difference is the STI!
> 
> So do the local_irq_enable(); mwait_idle_with_hints(0,0); thing.
> 

No, that doesn't work.  The point of __sti_mwait() is that the STI is
the instruction immediately before the MWAIT, just like the combination
STI;HLT.  Since the execution of STI is always delayed by one
instruction, these two instructions form an atomic unit, which means
interrupts are enabled "after" we have entered MWAIT or HLT.

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

If you think of STI;MWAIT as a "compound instruction" it kind of is.
Newer CPUs don't have to play that trick anymore, because there is a
flag to MWAIT which breaks us out of MWAIT on a pending interrupt
without having to actually enable interrupts at the point of the MWAIT.

	-hpa

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