RE: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose which counter to return
From: Jianyong Wu
Date: Mon May 25 2020 - 00:50:54 EST
> -----Original Message-----
> From: Richard Cochran <richardcochran@xxxxxxxxx>
> Sent: Sunday, May 24, 2020 10:11 AM
> To: Jianyong Wu <Jianyong.Wu@xxxxxxx>
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> Suzuki Poulose <Suzuki.Poulose@xxxxxxx>; Steven Price
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> kernel@xxxxxxxxxxxxxxxxxxx; kvmarm@xxxxxxxxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; Steve Capper <Steve.Capper@xxxxxxx>; Kaly Xin
> <Kaly.Xin@xxxxxxx>; Justin He <Justin.He@xxxxxxx>; Wei Chen
> <Wei.Chen@xxxxxxx>; nd <nd@xxxxxxx>
> 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.
> > 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.