linux-kernel - Re: [PATCH 1/2] x86: fix keeping track of AMD C1E

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Date:	Thu, 15 Jul 2010 23:56:40 +0200
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <borislav.petkov@....com>
CC:	Michal Schmidt <mschmidt@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	Shaohua Li <shaohua.li@...el.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/2] x86: fix keeping track of AMD C1E

On 07/16/2010 08:39 AM, Borislav Petkov wrote:
> From: "H. Peter Anvin"<hpa@...or.com>
> Date: Fri, Jul 16, 2010 at 12:25:39AM -0400
>
>> On 07/14/2010 02:31 PM, Michal Schmidt wrote:
>>>
>>> This suggests that another way to fix my problem would be this (tested):
>>>
>>
>> This is a change of semantics from an AND to an OR across CPUs...
>
> You mean the c1e_detected variable and the CPUID flag, right? Well,
> frankly and if I'm not missing anything, we actually only need to track
> when either bits [27,28] get set in that MSR - MSR_K8_INT_PENDING_MSG -
> in order to do timer broadcast.
>
> And strictly speaking, we don't need a variable for that at all (nor a
> synthetic CPUID flag, for that matter) - we can simply read the MSR as
> much as we'd like after we've detected that this CPU supports C1E.
>
> But having the value cached is faster and doesn't enlarge checking
> code in acpi_processor_cstate_check().
>
> I think the reason for adding the syntetic cpuid flag is only to
> communicate to the ACPI processor module that we don't support deeper
> C-states on a C1E machine, see a8d6829044901a67732904be5f1eacdf8539604f.
> So we don't strictly need it and we can only export c1e_detected to the
> rest for simplicity.
>

No, the difference between using a separate variable and the CPU feature 
bit is that CPU feature bit is ANDed across all CPUs, whereas this 
variable is set if it is set on *any* CPU.

	-hpa

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