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

From: Ingo Molnar <[email-masked]>
Date: Mon, 20 Jan 2014 10:29:31 +0100

* H. Peter Anvin <[email-masked]> wrote:

> 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.

As a side note, at minimum the semantic and compatibility difference 
needs to be _very_ clearly present in the naming. Something like 
mwait_old_() or mwait_core2_(). That way such dependencies and 
assumptions don't get lost in code restructuring, etc.

Thanks,

	Ingo