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Date:   Mon, 25 May 2020 10:17:18 +0100
From:   Marc Zyngier <>
To:     Richard Cochran <>,
        Jianyong Wu <>
Subject: Re: [RFC PATCH v12 10/11] arm64: add mechanism to let user choose
 which counter to return

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.

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.

For a Linux guest, we always know which reference we're using (the
virtual counter). So it is pointless to expose the choice to userspace
at all.

>> 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.
>> +		 * 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.

Not even that. If anything, the driver can obtain full knowledge of 
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.

Jazz is not dead. It just smells funny...

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