linux-kernel - Re: [PATCH] x86: limit mwait

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Message-Id: <200704051737.17157.ak@suse.de>
Date:	Thu, 5 Apr 2007 17:37:17 +0200
From:	Andi Kleen <ak@...e.de>
To:	"Andreas Herrmann" <andreas.herrmann3@....com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: limit mwait_idle to Intel CPUs

> > > This patch will enable default_idle for non-Intel
> > > CPUs even if mwait is supported.
> > 
> > It would be better to clear MONITOR/MWAIT in the AMD specific
> > CPU initialize code than add workarounds everywhere else.
> 
> Why is that?
> MONITOR/MWAIT is usable.

If it doesn't save power it's not usable imho.

> And I think this should 
> be indicated by cpuinfo.
> It's just inappropriate to use it in pm_idle.

There are no other users anyways and user space can't use it.
Ok in theory you could add a X86_FEATURE_MWAIT_DOESNT_SAVE_POWER
and check that, but just clearing it seems simpler and equivalent.

What would perhaps make sense is to add a idle=mwait command
line option for this though. So that the benchmarkers who currently
use idle=poll could migrate to idle=mwait. That option would need
to check the real cpuid bit of course again, but that should be
easy enough.

-Andi

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