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Date: Wed, 20 May 2015 10:55:20 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Borislav Petkov <bp@suse.de>
Cc: Huang Rui <ray.huang@amd.com>, Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Fengguang Wu <fengguang.wu@intel.com>,
        Aaron Lu <aaron.lu@intel.com>, Tony Li <tony.li@amd.com>
Subject: Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a
 configurable timer
Message-ID: <20150520085520.GA8566@gmail.com>
References: <1432022472-2224-1-git-send-email-ray.huang@amd.com>
 <1432022472-2224-3-git-send-email-ray.huang@amd.com>
 <20150519113121.GD4819@pd.tnic>
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* Borislav Petkov <bp@suse.de> wrote:

> On Tue, May 19, 2015 at 04:01:10PM +0800, Huang Rui wrote:
> > MWAITX/MWAIT does not let the cpu core go into C1 state on AMD processors.
> > The cpu core still consumes less power while waiting, and has faster exit
> > from waiting than "Halt". This patch implements an interface using the
> > kernel parameter "idle=" to configure mwaitx type and timer value.
> > 
> > If "idle=mwaitx", the timeout will be set as the maximum value
> > ((2^64 - 1) * TSC cycle).
> > If "idle=mwaitx,100", the timeout will be set as 100ns.
> > If the processor doesn't support MWAITX, then halt is used.

So what does the hardware do with the timeout value?

Does it use it to decide how 'deep' a sleep it will go into, i.e. 
larger timeouts cause longer entry and exit latencies?

Or some other purpose?

I suppose it's also the case that if an interrupt arrives _before_ the 
expected timeout then MWAITX will try to exit immediately, it won't 
wait until the timeout, right?

Thanks,

	Ingo