linux-kernel - Re: [tip:x86/idle] x86, hotplug: Move WBINVD back outside the play

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Message-ID: <4C97A5EF.4070100@zytor.com>
Date:	Mon, 20 Sep 2010 11:20:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	asit.k.mallick@...el.com, arjan@...ux.kernel.org,
	a.p.zijlstra@...llo.hl, lenb@...nel.org, tglx@...utronix.de,
	hpa@...ux.intel.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/idle] x86, hotplug: Move WBINVD back outside the play_dead
 loop

On 09/17/2010 05:48 PM, Venkatesh Pallipadi wrote:
>>
>> +       wbinvd();
>> +
>>        while (1) {
>> -               mb();
>> -               wbinvd();
>>                __monitor(&current_thread_info()->flags, 0, 0);
>>                mb();
> 
> 
> Just one observation. There are some CPUs with errata that need
> clflush before monitor. So, if that CPU wakesup spuriously it may have
> problem reentering idle. Not sure whether that will be a problem as
> that errata also depended on read happening on the flag. May be its
> better to do monitor (0, 0, 0).
> 

It seems the easy way to deal with that would be to just add clflush
before monitor... it is *probably* redundant, but it should be safe to
do. It means depending on X86_FEATURE_CLFLUSH as well as
X86_FEATURE_MWAIT, but I don't think there is any x86 processor which
has MWAIT and not CLFLUSH, and I highly doubt there ever will be.

Does anyone see any downside?

	-hpa
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