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From:   Jianyong Wu <Jianyong.Wu@arm.com>
To:     Marc Zyngier <maz@kernel.org>,
        Richard Cochran <richardcochran@gmail.com>
CC:     "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "yangbo.lu@nxp.com" <yangbo.lu@nxp.com>,
        "john.stultz@linaro.org" <john.stultz@linaro.org>,
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        Mark Rutland <Mark.Rutland@arm.com>,
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        Suzuki Poulose <Suzuki.Poulose@arm.com>,
        Steven Price <Steven.Price@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        "kvm@vger.kernel.org" <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
Thread-Topic: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose
 which counter to return
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Date:   Mon, 25 May 2020 14:18:18 +0000
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Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@kernel.org>
> Sent: Monday, May 25, 2020 5:17 PM
> To: Richard Cochran <richardcochran@gmail.com>; 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;
> 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
>=20
> On 2020-05-24 03:11, Richard Cochran wrote:
> > 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.
>=20
> There is no such thing as a distinction between nested or non-nested.
> Both counters are available to the guest at all times, and said guest can
> choose whichever it wants to use. So the hypervisor (KVM) has to support
> both counters as a reference.
>=20
It's great that we can decide which counter to return in guest kernel. So w=
e can abandon these code, including patch 9/11 and 10/11, that expose the i=
nterface to userspace to do the decision.

> For a Linux guest, we always know which reference we're using (the virtua=
l
> counter). So it is pointless to expose the choice to userspace at all.
>=20
So, we should throw these code of deciding counter type in linux driver awa=
y and just keep the hypercall service of providing both virtual counter and=
 physical counter in linux to server non-linux guest.
Am I right?

> >
> >> 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 =3D 0;
> >>
> >> +#ifdef CONFIG_ARM64
> >> +	static long flag;
> >
> > static?  This is not going to fly.
> >
> >> +		 * 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?
> >
> > From your description, this "flag" really should be a module
> > parameter.
>=20
> Not even that. If anything, the driver can obtain full knowledge of which
> counter is in use without any help. And the hard truth is that it is
> *always* the virtual counter as far as Linux is concerned.

Good!

Thanks
Jianyong=20
=20
>=20
>          M.
> --
> Jazz is not dead. It just smells funny...