linux-kernel - Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a configurable timer

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Message-ID: <20150520114125.GA31212@gmail.com>
Date:	Wed, 20 May 2015 13:41:25 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...e.de>
Cc:	Huang Rui <ray.huang@....com>, Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>, Tony Li <tony.li@....com>,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a
 configurable timer


* Borislav Petkov <bp@...e.de> wrote:

> On Wed, May 20, 2015 at 01:11:20PM +0200, Ingo Molnar wrote:
> >   - MWAITX takes a 'timeout' parameter, but otherwise behaves exactly 
> >     like MWAIT: i.e. once idle it won't exit idle on its own
> 
> Let me quote the commit message:
> 
> "MWAITT, another name is MWAITX (MWAIT with extensions), has a
> configurable timer that causes MWAITX to exit on expiration."

Ah. A useful skill that is, being able to read.

> You need to set the second bit in ECX to enable the timer.
> 
> I guess if you don't, then you get normal MWAIT but then you don't 
> need the timeout either...

Yeah.

So if it's a true timeout then we could use it to implement irq-less 
timers: that's actually pretty useful, because it could be faster than 
getting a local APIC timer irq, etc.

Thanks,

	Ingo
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