linux-kernel - Re: [PATCH 1/2] x86/xen: sync the wallclock when the system time changes

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Date:	Thu, 30 May 2013 17:30:50 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	xen-devel@...ts.xen.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/xen: sync the wallclock when the system time
 changes

On 05/30/2013 07:25 AM, David Vrabel wrote:
> From: David Vrabel <david.vrabel@...rix.com>
>
> Currently the Xen wallclock is only updated every 11 minutes if NTP is
> synchronized to its clock source.  If a guest is started before NTP is
> synchronized it may see an incorrect wallclock time.

Ok.. So this is maybe starting to make a little more sense.

I was under the mistaken impression domN guests referenced dom0's system 
time when they called xen_get_wallclock(), but looking at this a bit 
closer, its starting to make a bit more sense that xen_get_wallclock() 
is just shared hypervisor data that is updated by dom0, and guests can 
access this data without interacting with dom0.

Thus I can finally see the 11 minute update interval for dom0 to update 
the hypervisor wallclock data causes guests to get invalid time values 
when they initialize, reading the unset by dom0 hypervisor wallclock 
data. And thus I finally see the need for dom0 to more frequently update 
the hypervisor wallclock data.

So first, sorry for being so dense through all of this. But this really 
could use a clearer explanation as to the nature of the issue.

Few minor issues below.

> Use the pvclock_gtod notifier chain to receive a notification when the
> system time has changed and update the wallclock to match.
>
> This chain is called on every timer tick and we want to avoid an extra
> (expensive) hypercall on every tick.  Because dom0 has historically
> never provided a very accurate wallclock and guests do not expect one,
> we can do this simply.  The wallclock is only updated if the
> difference between now and the last update is more than 0.5 s.

So given (at least I think ) I get why this is needed, is there a reason 
you're using the notifier chain instead of a regular timer in dom0 to 
update the hypervisor's wallclock data?

>
> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
> ---
>   arch/x86/xen/time.c |   46 ++++++++++++++++++++++++++++++++++++++++++++++
>   1 files changed, 46 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index a1947ac..60b7d1f 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -14,6 +14,7 @@
>   #include <linux/kernel_stat.h>
>   #include <linux/math64.h>
>   #include <linux/gfp.h>
> +#include <linux/pvclock_gtod.h>
>   
>   #include <asm/pvclock.h>
>   #include <asm/xen/hypervisor.h>
> @@ -212,6 +213,48 @@ static int xen_set_wallclock(const struct timespec *now)
>   	return HYPERVISOR_dom0_op(&op);
>   }
>   
> +static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
> +				   void *priv)
> +{
> +	static struct timespec last, next;
> +	struct timespec now;
> +	struct xen_platform_op op;
> +	int ret;
> +
> +	/*
> +	 * Set the Xen wallclock from Linux system time.
> +	 *
> +	 * dom0 hasn't historically maintained a very accurate
> +	 * wallclock so guests don't expect it. We can therefore
> +	 * reduce the number of expensive hypercalls by only updating
> +	 * the wallclock every 0.5 s.

This comment needs some improvement. It doesn't explain why Xen needs to 
set the virtual RTC so frequently, but then goes on to say it can be 
done every half second because guests don't really expect it anyway.

> +	 */
> +
> +	now = __current_kernel_time();

You don't seem to be holding the timekeeping lock here, so why are you 
calling the internal __ prefixed current_kernel_time() accessor?

> +
> +	if (timespec_compare(&now, &last) > 0

Not sure I understand why you're bothering with the last value? Aren't 
you just wanting to trigger when now is greater then next?

So again, apologies for some of the runaround in the discussion! Lets 
sort out the above minor issues and I'll be fine to queue this (given 
Xen maintainer acks) without grumbling.

thanks
-john

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