linux-kernel - RE: [PATCH] x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on AMD systems

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Date:   Tue, 23 May 2017 12:50:15 +0000
From:   "Ghannam, Yazen" <Yazen.Ghannam@....com>
To:     Borislav Petkov <bp@...en8.de>
CC:     "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "len.brown@...el.com" <len.brown@...el.com>,
        "pavel@....cz" <pavel@....cz>
Subject: RE: [PATCH] x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on AMD
 systems

> -----Original Message-----
> From: Borislav Petkov [mailto:bp@...en8.de]
> Sent: Tuesday, May 23, 2017 3:59 AM
> To: Ghannam, Yazen <Yazen.Ghannam@....com>
> Cc: linux-pm@...r.kernel.org; x86@...nel.org; linux-
> kernel@...r.kernel.org; rjw@...ysocki.net; len.brown@...el.com;
> pavel@....cz
> Subject: Re: [PATCH] x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on
> AMD systems
> 
> On Mon, May 22, 2017 at 08:20:56PM +0000, Ghannam, Yazen wrote:
> > On the other hand, MWAIT on AMD limits the hardware to using only
> > certain, shallower Cstates. This is okay if we define individual
> > states and use MWAIT for some of them. But it would consume more
> power if used always.
> 
> Let me see if I understand it correctly:
> 
> Even though we used to do HLT on previous families as idling with HLT
> *is* the preferred method until now, with your change you're moving
> *every* AMD machine out there to do MWAIT now.
> 

No, AMD systems will continue to use HLT unless the BIOS specifies the
use of MWAIT using a FFH entry in the ACPI _CST.

All this change does is *allow* us to use MWAIT through the FFH
implementation if the BIOS defines it. It doesn't *force* a change.

If the BIOS doesn't define the appropriate _CST entry or it defines it
wrong, then we'll fallback to using HLT.

Thanks,
Yazen