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

From: "Ghannam, Yazen" <Yazen.Ghannam@amd.com>
To: Borislav Petkov <bp@alien8.de>
Cc: "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"rjw@rjwysocki.net" <rjw@rjwysocki.net>,
	"len.brown@intel.com" <len.brown@intel.com>,
	"pavel@ucw.cz" <pavel@ucw.cz>
Subject: RE: [PATCH] x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on AMD systems
Date: Tue, 23 May 2017 12:50:15 +0000	[thread overview]
Message-ID: <BN6PR1201MB0131B6E13B6E782494751054F8F90@BN6PR1201MB0131.namprd12.prod.outlook.com> (raw)
In-Reply-To: <20170523075911.GA25383@nazgul.tnic>

> -----Original Message-----
> From: Borislav Petkov [mailto:bp@alien8.de]
> Sent: Tuesday, May 23, 2017 3:59 AM
> To: Ghannam, Yazen <Yazen.Ghannam@amd.com>
> Cc: linux-pm@vger.kernel.org; x86@kernel.org; linux-
> kernel@vger.kernel.org; rjw@rjwysocki.net; len.brown@intel.com;
> pavel@ucw.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