Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in shutdown

[Wei Liu <wei.liu@kernel.org>]

On Wed, Apr 12, 2023 at 12:02:04PM +0000, Li,Rongqing wrote:
> 
> 
> > -----Original Message-----
> > From: Sean Christopherson <seanjc@google.com>
> > Sent: Wednesday, February 8, 2023 11:04 PM
> > To: Michael Kelley (LINUX) <mikelley@microsoft.com>
> > Cc: Li,Rongqing <lirongqing@baidu.com>; KY Srinivasan <kys@microsoft.com>;
> > Haiyang Zhang <haiyangz@microsoft.com>; wei.liu@kernel.org; Dexuan Cui
> > <decui@microsoft.com>; tglx@linutronix.de; mingo@redhat.com;
> > bp@alien8.de; dave.hansen@linux.intel.com; x86@kernel.org;
> > linux-hyperv@vger.kernel.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in
> > shutdown
> > 
> > On Wed, Feb 08, 2023, Michael Kelley (LINUX) wrote:
> > > From: lirongqing@baidu.com <lirongqing@baidu.com> Sent: Monday,
> > > February 6, 2023 5:15 PM
> > > >
> > > > Zeroing the counter register in pit_shutdown() isn't actually
> > > > supposed to stop it from counting,  will causes the PIT to start
> > > > running again, From the spec:
> > > >
> > > >   The largest possible initial count is 0; this is equivalent to 216 for
> > > >   binary counting and 104 for BCD counting.
> > > >
> > > >   The Counter does not stop when it reaches zero. In Modes 0, 1, 4, and 5
> > the
> > > >   Counter "wraps around" to the highest count, either FFFF hex for binary
> > > >   count- ing or 9999 for BCD counting, and continues counting.
> > > >
> > > >   Mode 0 is typically used for event counting. After the Control Word is
> > > >   written, OUT is initially low, and will remain low until the Counter
> > > >   reaches zero. OUT then goes high and remains high until a new count or a
> > > >   new Mode 0 Control Word is written into the Counter.
> > > >
> > > > Hyper-V and KVM follow the spec, the issue that 35b69a42
> > > > "(clockevents/drivers/
> > > > i8253: Add support for PIT shutdown quirk") fixed is in i8253
> > > > drivers, not Hyper-v, so delete the zero timer counter register in
> > > > shutdown, and delete PIT shutdown quirk for Hyper-v
> > >
> > > From the standpoint of Hyper-V, I'm good with this change.  But
> > > there's a risk that old hardware might not be compliant with the spec,
> > > and needs the zero'ing for some reason. The experts in the x86 space
> > > will be in the best position to assess the risk.
> > 
> > Yep, my feeling exactly.  My input is purely from reading those crusty old specs.
> 
> 
> 
> Ping

I guess you want Thomas Gleixner and Daniel Lezcano's attention.

> 
> Thanks
> 
> -Li