linux-kernel - RE: [PATCH] x86/smpboot: Don't do mwait_play

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Date:   Tue, 3 Apr 2018 13:55:36 +0000
From:   "Ghannam, Yazen" <Yazen.Ghannam@....com>
To:     Ingo Molnar <mingo@...nel.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...e.de" <bp@...e.de>
Subject: RE: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD systems

> -----Original Message-----
> From: Ingo Molnar <mingo.kernel.org@...il.com> On Behalf Of Ingo Molnar
> Sent: Tuesday, April 3, 2018 7:04 AM
> To: Ghannam, Yazen <Yazen.Ghannam@....com>
> Cc: x86@...nel.org; linux-kernel@...r.kernel.org; bp@...e.de
> Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD
> systems
> 
> 
> * Yazen Ghannam <Yazen.Ghannam@....com> wrote:
> 
> > From: Yazen Ghannam <yazen.ghannam@....com>
> >
> > Recent AMD systems support using MWAIT for C1 state. However, MWAIT will
> > not allow deeper cstates than C1 on current systems.
> >
> > With play_dead() we expect the OS to use the deepest state available.
> > The deepest state available on AMD systems is reached through SystemIO
> > or HALT. If MWAIT is available, we use it instead of the other methods,
> > so we never reach the deepest state.
> >
> > Don't try to use MWAIT to play_dead() on AMD systems. Instead, we'll use
> > CPUIDLE to enter the deepest state advertised by firmware. If CPUIDLE is
> > not available then we fallback to HALT.
> >
> > Signed-off-by: Yazen Ghannam <yazen.ghannam@....com>
> > ---
> >  arch/x86/kernel/smpboot.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > index ff99e2b6fc54..67cf00b25f83 100644
> > --- a/arch/x86/kernel/smpboot.c
> > +++ b/arch/x86/kernel/smpboot.c
> > @@ -1536,6 +1536,9 @@ static inline void mwait_play_dead(void)
> >  	void *mwait_ptr;
> >  	int i;
> >
> > +	/* Don't try native MWAIT on AMD. Stick to CPUIDLE and HALT. */
> > +	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> > +		return;
> 
> The comment should mainly explain the 'why is this done', not the 'what is done'
> which is pretty obvious from the code ...
> 

Yes, I'll drop that comment since the commit message has the explanation.

Thanks,
Yazen