linux-kernel - Re: [PATCH] x86: Voluntary leave

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Message-ID: <476A9576.3020203@zytor.com>
Date:	Thu, 20 Dec 2007 08:16:54 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Len Brown <lenb@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Voluntary leave_mm before entering ACPI C3

Arjan van de Ven wrote:
> On Wed, 19 Dec 2007 11:48:14 -0800
> "H. Peter Anvin" <hpa@...or.com> wrote:
> 
>> I think C3 guarantees that the cache contents stay intact, and thus
>> it might make sense in some technology to preserve the TLB as well
>> (being a kind of cache.)
> 
> that sounds nice. It's fiction though ;-)
> 
> The thing to realize is that linux only sees "ACPI C3"; the BIOS maps that C3 to.. well any of the C states the processor in the system has. What you're saying is afaik correct for the *hardware* C3, not for the "C3" that Linux sees..
> 

Well, it can only map ACPI C3 to a state which is no more "dead" than 
what would normally be permitted by C3.  IIRC, C3 is allowed to require 
that DMA be turned off (unlike C2), but is not allowed to lose the CPU 
state.

	-hpa
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