RE: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose which counter to return

[Jianyong Wu <Jianyong.Wu@arm.com>]

Hi Richard,

> -----Original Message-----
> From: Richard Cochran <richardcochran@gmail.com>
> Sent: Sunday, May 24, 2020 10:11 AM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org;
> tglx@linutronix.de; pbonzini@redhat.com; sean.j.christopherson@intel.com;
> maz@kernel.org; Mark Rutland <Mark.Rutland@arm.com>; will@kernel.org;
> Suzuki Poulose <Suzuki.Poulose@arm.com>; Steven Price
> <Steven.Price@arm.com>; linux-kernel@vger.kernel.org; linux-arm-
> kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> kvm@vger.kernel.org; Steve Capper <Steve.Capper@arm.com>; Kaly Xin
> <Kaly.Xin@arm.com>; Justin He <Justin.He@arm.com>; Wei Chen
> <Wei.Chen@arm.com>; nd <nd@arm.com>
> Subject: Re: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose
> which counter to return
> 
> On Fri, May 22, 2020 at 04:37:23PM +0800, Jianyong Wu wrote:
> > In general, vm inside will use virtual counter compered with host use
> > phyical counter. But in some special scenarios, like nested
> > virtualization, phyical counter maybe used by vm. A interface added in
> > ptp_kvm driver to offer a mechanism to let user choose which counter
> > should be return from host.
> 
> Sounds like you have two time sources, one for normal guest, and one for
> nested.  Why not simply offer the correct one to user space automatically?  If
> that cannot be done, then just offer two PHC devices with descriptive names.
> 

It's a good idea, but in most case physical counter will not be used, so it's  better not keep 2 ptp devices all the time.
How about adding an extra argument in struct ptp_clock_info to serve as a flag, then we can control this flag using IOCTL to determine the counter type.
In this way, no extra arguments needed in .getcrosststamp. But we also need specific code in ptp_ioctl to implement it like in this patch.

The second way, maybe we can use the flag as a module parameter, this is easier to implement.
  @maz@kernel.org WDYT?
 
> > diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
> > index fef72f29f3c8..8b0a7b328bcd 100644
> > --- a/drivers/ptp/ptp_chardev.c
> > +++ b/drivers/ptp/ptp_chardev.c
> > @@ -123,6 +123,9 @@ long ptp_ioctl(struct posix_clock *pc, unsigned int
> cmd, unsigned long arg)
> >  	struct timespec64 ts;
> >  	int enable, err = 0;
> >
> > +#ifdef CONFIG_ARM64
> > +	static long flag;
> 
> static?  This is not going to fly.

Need remove here.

> 
> > +		 * In most cases, we just need virtual counter from host and
> > +		 * there is limited scenario using this to get physical counter
> > +		 * in guest.
> > +		 * Be careful to use this as there is no way to set it back
> > +		 * unless you reinstall the module.
> 
> How on earth is the user supposed to know this?
> 
Yeah, It's odd , should be removed.

> From your description, this "flag" really should be a module parameter.
Maybe use flag as a module parameter is a better way.

Thanks
Jianyong 
> 
> Thanks,
> Richard