> -----Original Message----- > From: Ingo Molnar <mingo.kernel....@gmail.com> On Behalf Of Ingo Molnar > Sent: Tuesday, April 3, 2018 10:41 AM > To: Ghannam, Yazen <yazen.ghan...@amd.com> > Cc: x...@kernel.org; linux-kernel@vger.kernel.org; b...@suse.de > Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD > systems > > > * Ghannam, Yazen <yazen.ghan...@amd.com> wrote: > > > > -----Original Message----- > > > From: Ingo Molnar <mingo.kernel....@gmail.com> On Behalf Of Ingo > Molnar > > > Sent: Tuesday, April 3, 2018 7:04 AM > > > To: Ghannam, Yazen <yazen.ghan...@amd.com> > > > Cc: x...@kernel.org; linux-kernel@vger.kernel.org; b...@suse.de > > > Subject: Re: [PATCH] x86/smpboot: Don't do mwait_play_dead() on AMD > > > systems > > > > > > > > > * Yazen Ghannam <yazen.ghan...@amd.com> wrote: > > > > > > > From: Yazen Ghannam <yazen.ghan...@amd.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.ghan...@amd.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. > > Or rather, explain the 'why' in the comment, because otherwise this is a > pretty > obscure condition that is not self-documenting? >
Okay, how about this? "MWAIT only provides shallow Cstates on AMD systems. Fallback to CPUIDLE and HALT to have access to deeper Cstates." Thanks, Yazen
