[PATCHv2 0/2] xen: maintain an accurate persistent clock in more cases

[PATCHv2 0/2] xen: maintain an accurate persistent clock in more cases

[David Vrabel]

The kernel has limited support for updating the persistent clock or
RTC when NTP is synced.  This has the following limitations:

* The persistent clock is not updated on step changes.  This leaves a
  window where it will be incorrect (while NTP resyncs).

* Xen guests use the Xen wallclock as their persistent clock.  dom0
  maintains this clock so it is persistent for domUs and not dom0
  itself.

These series fixes the above limitations and depends on "x86: increase
precision of x86_platform.get/set_wallclock()" which was previously
posted.

Changes since v1:

Reworked to use the pvclock_gtod notifier to sync the wallclock (this
looked similar to what a KVM host does).  update_persistent_clock()
will now only update the CMOS RTC.

David

[1] http://lists.xen.org/archives/html/xen-devel/2013-05/msg01402.html

[PATCH 1/2] x86/xen: sync the CMOS RTC as well as the Xen wallclock

[David Vrabel]

From: David Vrabel <david.vrabel@citrix.com>

Adjustments to Xen's persistent_clock via update_persistent_clock()
don't actually persist, as the xen_set_walltime() just notifies other
domN guests that it has been updated, and does not modify the
underlying CMOS clock.

Thus, this patch modifies xen_set_wallclock() so it will set the
underlying CMOS clock when called from dom0, ensuring the
persistent_clock will be correct on the next hardware boot.

Dom0 does not support accessing EFI runtime services and Xen does not
run on Moorsetown platforms so the CMOS RTC is the only supported
hardware clock.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 arch/x86/xen/time.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index a1947ac..4656165 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/mc146818rtc.h>
 
 #include <asm/pvclock.h>
 #include <asm/xen/hypervisor.h>
@@ -199,17 +200,25 @@ static void xen_get_wallclock(struct timespec *now)
 static int xen_set_wallclock(const struct timespec *now)
 {
 	struct xen_platform_op op;
+	int ret;
 
 	/* do nothing for domU */
 	if (!xen_initial_domain())
 		return -1;
 
+	/* Set the Xen wallclock. */
 	op.cmd = XENPF_settime;
 	op.u.settime.secs = now->tv_sec;
 	op.u.settime.nsecs = now->tv_nsec;
 	op.u.settime.system_time = xen_clocksource_read();
 
-	return HYPERVISOR_dom0_op(&op);
+	ret = HYPERVISOR_dom0_op(&op);
+	if (ret)
+		return ret;
+
+	/* Set the hardware RTC. */
+	return mach_set_rtc_mmss(now);
+
 }
 
 static struct clocksource xen_clocksource __read_mostly = {
-- 
1.7.2.5

Re: [PATCH 1/2] x86/xen: sync the CMOS RTC as well as the Xen wallclock

[John Stultz]

On 05/28/2013 11:22 AM, David Vrabel wrote:
> From: David Vrabel <david.vrabel@citrix.com>
>
> Adjustments to Xen's persistent_clock via update_persistent_clock()
> don't actually persist, as the xen_set_walltime() just notifies other
> domN guests that it has been updated, and does not modify the
> underlying CMOS clock.
>
> Thus, this patch modifies xen_set_wallclock() so it will set the
> underlying CMOS clock when called from dom0, ensuring the
> persistent_clock will be correct on the next hardware boot.
>
> Dom0 does not support accessing EFI runtime services and Xen does not
> run on Moorsetown platforms so the CMOS RTC is the only supported
> hardware clock.
>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Ok, I've queued this for 3.11.

thanks
-john

Re: [PATCH 1/2] x86/xen: sync the CMOS RTC as well as the Xen wallclock

[Konrad Rzeszutek Wilk]

On Tue, May 28, 2013 at 12:01:57PM -0700, John Stultz wrote:
> On 05/28/2013 11:22 AM, David Vrabel wrote:
> >From: David Vrabel <david.vrabel@citrix.com>
> >
> >Adjustments to Xen's persistent_clock via update_persistent_clock()
> >don't actually persist, as the xen_set_walltime() just notifies other
> >domN guests that it has been updated, and does not modify the
> >underlying CMOS clock.
> >
> >Thus, this patch modifies xen_set_wallclock() so it will set the
> >underlying CMOS clock when called from dom0, ensuring the
> >persistent_clock will be correct on the next hardware boot.
> >
> >Dom0 does not support accessing EFI runtime services and Xen does not
> >run on Moorsetown platforms so the CMOS RTC is the only supported
> >hardware clock.
> >
> >Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> >Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> 
> Ok, I've queued this for 3.11.

Thanks.
> 
> thanks
> -john
> 

[PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[David Vrabel]

From: David Vrabel <david.vrabel@citrix.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.

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.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 arch/x86/xen/time.c |   54 ++++++++++++++++++++++++++++++++++++++++++--------
 1 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 4656165..81027b5 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -15,6 +15,8 @@
 #include <linux/math64.h>
 #include <linux/gfp.h>
 #include <linux/mc146818rtc.h>
+#include <linux/timekeeper_internal.h>
+#include <linux/pvclock_gtod.h>
 
 #include <asm/pvclock.h>
 #include <asm/xen/hypervisor.h>
@@ -199,28 +201,59 @@ static void xen_get_wallclock(struct timespec *now)
 
 static int xen_set_wallclock(const struct timespec *now)
 {
-	struct xen_platform_op op;
-	int ret;
-
 	/* do nothing for domU */
 	if (!xen_initial_domain())
 		return -1;
 
-	/* Set the Xen wallclock. */
+	/* Set the hardware RTC. */
+	return mach_set_rtc_mmss(now);
+
+}
+
+static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
+				   void *priv)
+{
+	static struct timespec last, next;
+	struct timespec now;
+	struct timekeeper *tk = priv;
+	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.
+	 */
+
+	now.tv_sec = tk->xtime_sec;
+	now.tv_nsec = tk->xtime_nsec >> tk->shift;
+
+	if (timespec_compare(&now, &last) > 0
+	    && timespec_compare(&now, &next) < 0)
+		return 0;
+
 	op.cmd = XENPF_settime;
-	op.u.settime.secs = now->tv_sec;
-	op.u.settime.nsecs = now->tv_nsec;
+	op.u.settime.secs = now.tv_sec;
+	op.u.settime.nsecs = now.tv_nsec;
 	op.u.settime.system_time = xen_clocksource_read();
 
 	ret = HYPERVISOR_dom0_op(&op);
 	if (ret)
-		return ret;
+		return 0;
 
-	/* Set the hardware RTC. */
-	return mach_set_rtc_mmss(now);
+	last = now;
+	next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
 
+	return 0;
 }
 
+static struct notifier_block xen_pvclock_gtod_notifier = {
+	.notifier_call = xen_pvclock_gtod_notify,
+};
+
 static struct clocksource xen_clocksource __read_mostly = {
 	.name = "xen",
 	.rating = 400,
@@ -482,6 +515,9 @@ static void __init xen_time_init(void)
 	xen_setup_runstate_info(cpu);
 	xen_setup_timer(cpu);
 	xen_setup_cpu_clockevents();
+
+	if (xen_initial_domain())
+		pvclock_gtod_register_notifier(&xen_pvclock_gtod_notifier);
 }
 
 void __init xen_init_time_ops(void)
-- 
1.7.2.5

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

[John Stultz]

On 05/28/2013 11:22 AM, David Vrabel wrote:
> From: David Vrabel <david.vrabel@citrix.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.
>
> 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.
>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> ---
>   arch/x86/xen/time.c |   54 ++++++++++++++++++++++++++++++++++++++++++--------
>   1 files changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 4656165..81027b5 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -15,6 +15,8 @@
>   #include <linux/math64.h>
>   #include <linux/gfp.h>
>   #include <linux/mc146818rtc.h>
> +#include <linux/timekeeper_internal.h>

Nope. You shouldn't be including this.

> +#include <linux/pvclock_gtod.h>
>   
>   #include <asm/pvclock.h>
>   #include <asm/xen/hypervisor.h>
> @@ -199,28 +201,59 @@ static void xen_get_wallclock(struct timespec *now)
>   
>   static int xen_set_wallclock(const struct timespec *now)
>   {
> -	struct xen_platform_op op;
> -	int ret;
> -
>   	/* do nothing for domU */
>   	if (!xen_initial_domain())
>   		return -1;
>   
> -	/* Set the Xen wallclock. */
> +	/* Set the hardware RTC. */
> +	return mach_set_rtc_mmss(now);
> +
> +}
> +
> +static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
> +				   void *priv)
> +{
> +	static struct timespec last, next;
> +	struct timespec now;
> +	struct timekeeper *tk = priv;

Noooo.. don't export the timekeeper like that.


> +	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.
> +	 */
> +
> +	now.tv_sec = tk->xtime_sec;
> +	now.tv_nsec = tk->xtime_nsec >> tk->shift;

You probably want current_kernel_time() here.

> +
> +	if (timespec_compare(&now, &last) > 0
> +	    && timespec_compare(&now, &next) < 0)
> +		return 0;
> +
>   	op.cmd = XENPF_settime;
> -	op.u.settime.secs = now->tv_sec;
> -	op.u.settime.nsecs = now->tv_nsec;
> +	op.u.settime.secs = now.tv_sec;
> +	op.u.settime.nsecs = now.tv_nsec;
>   	op.u.settime.system_time = xen_clocksource_read();
>   
>   	ret = HYPERVISOR_dom0_op(&op);
>   	if (ret)
> -		return ret;
> +		return 0;
>   
> -	/* Set the hardware RTC. */
> -	return mach_set_rtc_mmss(now);
> +	last = now;
> +	next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
>   

Am I missing the xen_set_wallclock hook here? Your previous patch wanted 
to call the dom0 op and then set the hardware RTC.

And this notifier get called every timer tick? This seems out there...

thanks
-john

Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[Jan Beulich]

>>> On 28.05.13 at 20:53, John Stultz <john.stultz@linaro.org> wrote:
> On 05/28/2013 11:22 AM, David Vrabel wrote:
>>   	op.cmd = XENPF_settime;
>> -	op.u.settime.secs = now->tv_sec;
>> -	op.u.settime.nsecs = now->tv_nsec;
>> +	op.u.settime.secs = now.tv_sec;
>> +	op.u.settime.nsecs = now.tv_nsec;
>>   	op.u.settime.system_time = xen_clocksource_read();
>>   
>>   	ret = HYPERVISOR_dom0_op(&op);
>>   	if (ret)
>> -		return ret;
>> +		return 0;
>>   
>> -	/* Set the hardware RTC. */
>> -	return mach_set_rtc_mmss(now);
>> +	last = now;
>> +	next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
>>   
> 
> Am I missing the xen_set_wallclock hook here? Your previous patch wanted 
> to call the dom0 op and then set the hardware RTC.

That ought to be intentional: We should update the hypervisor's
software wall clock here, but limit updating of the RTC to whenever
a native kernel would do this (i.e. among other things subject to
CONFIG_RTC_SYSTOHC).

Jan

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

[David Vrabel]

On 28/05/13 19:53, John Stultz wrote:
> On 05/28/2013 11:22 AM, David Vrabel wrote:
>> From: David Vrabel <david.vrabel@citrix.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.
>>
>> 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.
[...]
>> index 4656165..81027b5 100644
>> --- a/arch/x86/xen/time.c
>> +++ b/arch/x86/xen/time.c
[...]
>> +    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.
>> +     */
>> +
>> +    now.tv_sec = tk->xtime_sec;
>> +    now.tv_nsec = tk->xtime_nsec >> tk->shift;
> 
> You probably want current_kernel_time() here.

Ah. I was looking for something like this but since the notifier chain
is called with various locks held stuff kept deadlocking.  It looks like
I can use __current_kernel_time() though.

>> +
>> +    if (timespec_compare(&now, &last) > 0
>> +        && timespec_compare(&now, &next) < 0)
>> +        return 0;
>> +
>>       op.cmd = XENPF_settime;
>> -    op.u.settime.secs = now->tv_sec;
>> -    op.u.settime.nsecs = now->tv_nsec;
>> +    op.u.settime.secs = now.tv_sec;
>> +    op.u.settime.nsecs = now.tv_nsec;
>>       op.u.settime.system_time = xen_clocksource_read();
>>         ret = HYPERVISOR_dom0_op(&op);
>>       if (ret)
>> -        return ret;
>> +        return 0;
>>   -    /* Set the hardware RTC. */
>> -    return mach_set_rtc_mmss(now);
>> +    last = now;
>> +    next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
>>   
> 
> Am I missing the xen_set_wallclock hook here? Your previous patch wanted
> to call the dom0 op and then set the hardware RTC.

As Jan says, this is deliberate.  We now update the hardware CMOS RTC in
the same way as bare metal and then use this notifier to only maintain
the Xen wallclock for guests.

> And this notifier get called every timer tick? This seems out there...

This notifier was added to support similar functionality for KVM and I'm
just reusing what's there.

See e0b306fef (time: export time information for KVM pvclock) [1] and
16e8d74d2 (KVM: x86: notifier for clocksource changes) [2].

Perhaps Marcelo can comment on why it was necessary for this to be
called on every timer tick.

David

[1]
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=e0b306fef90556233797d2e1747bd6a3ae35ea93
[2]
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=16e8d74d2da9920f874b10a3d979fb25c01f518f

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

[Marcelo Tosatti]

On Wed, May 29, 2013 at 10:42:04AM +0100, David Vrabel wrote:
> On 28/05/13 19:53, John Stultz wrote:
> > On 05/28/2013 11:22 AM, David Vrabel wrote:
> >> From: David Vrabel <david.vrabel@citrix.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.
> >>
> >> 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.
> [...]
> >> index 4656165..81027b5 100644
> >> --- a/arch/x86/xen/time.c
> >> +++ b/arch/x86/xen/time.c
> [...]
> >> +    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.
> >> +     */
> >> +
> >> +    now.tv_sec = tk->xtime_sec;
> >> +    now.tv_nsec = tk->xtime_nsec >> tk->shift;
> > 
> > You probably want current_kernel_time() here.
> 
> Ah. I was looking for something like this but since the notifier chain
> is called with various locks held stuff kept deadlocking.  It looks like
> I can use __current_kernel_time() though.
> 
> >> +
> >>  +    if (timespec_compare(&now, &last) > 0
> >> +        && timespec_compare(&now, &next) < 0)
> >> +        return 0;
> >> +
> >>       op.cmd = XENPF_settime;
> >> -    op.u.settime.secs = now->tv_sec;
> >> -    op.u.settime.nsecs = now->tv_nsec;
> >> +    op.u.settime.secs = now.tv_sec;
> >> +    op.u.settime.nsecs = now.tv_nsec;
> >>       op.u.settime.system_time = xen_clocksource_read();
> >>         ret = HYPERVISOR_dom0_op(&op);
> >>       if (ret)
> >> -        return ret;
> >> +        return 0;
> >>   -    /* Set the hardware RTC. */
> >> -    return mach_set_rtc_mmss(now);
> >> +    last = now;
> >> +    next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
> >>   
> > 
> > Am I missing the xen_set_wallclock hook here? Your previous patch wanted
> > to call the dom0 op and then set the hardware RTC.
> 
> As Jan says, this is deliberate.  We now update the hardware CMOS RTC in
> the same way as bare metal and then use this notifier to only maintain
> the Xen wallclock for guests.
> 
> > And this notifier get called every timer tick? This seems out there...
> 
> This notifier was added to support similar functionality for KVM and I'm
> just reusing what's there.
> 
> See e0b306fef (time: export time information for KVM pvclock) [1] and
> 16e8d74d2 (KVM: x86: notifier for clocksource changes) [2].
> 
> Perhaps Marcelo can comment on why it was necessary for this to be
> called on every timer tick.

Just so to maintain similarity with the vsyscall notifier (as in updates
all fields the vsyscall notifier uses).

KVM only actually makes use of the clocksource change case.

Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[Jan Beulich]

>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@citrix.com> wrote:
> +static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
> +				   void *priv)
> +{
> +	static struct timespec last, next;
> +	struct timespec now;
> +	struct timekeeper *tk = priv;
> +	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.
> +	 */
> +
> +	now.tv_sec = tk->xtime_sec;
> +	now.tv_nsec = tk->xtime_nsec >> tk->shift;
> +
> +	if (timespec_compare(&now, &last) > 0
> +	    && timespec_compare(&now, &next) < 0)

Is this really working the first time through (when both last and
next are still all zeros)?

Jan

> +		return 0;
> +
>  	op.cmd = XENPF_settime;
> -	op.u.settime.secs = now->tv_sec;
> -	op.u.settime.nsecs = now->tv_nsec;
> +	op.u.settime.secs = now.tv_sec;
> +	op.u.settime.nsecs = now.tv_nsec;
>  	op.u.settime.system_time = xen_clocksource_read();
>  
>  	ret = HYPERVISOR_dom0_op(&op);
>  	if (ret)
> -		return ret;
> +		return 0;
>  
> -	/* Set the hardware RTC. */
> -	return mach_set_rtc_mmss(now);
> +	last = now;
> +	next = timespec_add(now, ns_to_timespec(NSEC_PER_SEC / 2));
>  
> +	return 0;
>  }

Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[David Vrabel]

On 29/05/13 08:39, Jan Beulich wrote:
>>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@citrix.com> wrote:
>> +static int xen_pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
>> +				   void *priv)
>> +{
>> +	static struct timespec last, next;
>> +	struct timespec now;
>> +	struct timekeeper *tk = priv;
>> +	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.
>> +	 */
>> +
>> +	now.tv_sec = tk->xtime_sec;
>> +	now.tv_nsec = tk->xtime_nsec >> tk->shift;
>> +
>> +	if (timespec_compare(&now, &last) > 0
>> +	    && timespec_compare(&now, &next) < 0)
> 
> Is this really working the first time through (when both last and
> next are still all zeros)?

Yes. The first time through we want to set the wallclock and this test
is always false when last == next.

David

Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[Jan Beulich]

>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@citrix.com> wrote:
> @@ -199,28 +201,59 @@ static void xen_get_wallclock(struct timespec *now)
>  
>  static int xen_set_wallclock(const struct timespec *now)
>  {
> -	struct xen_platform_op op;
> -	int ret;
> -
>  	/* do nothing for domU */
>  	if (!xen_initial_domain())
>  		return -1;
>  
> -	/* Set the Xen wallclock. */
> +	/* Set the hardware RTC. */
> +	return mach_set_rtc_mmss(now);
> +
> +}

Can't you achieve the same effect in a cleaner way by overriding
x86_platform.set_wallclock only in the non-init-domain case?

Jan

Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[David Vrabel]

On 29/05/13 08:47, Jan Beulich wrote:
>>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@citrix.com> wrote:
>> @@ -199,28 +201,59 @@ static void xen_get_wallclock(struct timespec *now)
>>  
>>  static int xen_set_wallclock(const struct timespec *now)
>>  {
>> -	struct xen_platform_op op;
>> -	int ret;
>> -
>>  	/* do nothing for domU */
>>  	if (!xen_initial_domain())
>>  		return -1;
>>  
>> -	/* Set the Xen wallclock. */
>> +	/* Set the hardware RTC. */
>> +	return mach_set_rtc_mmss(now);
>> +
>> +}
> 
> Can't you achieve the same effect in a cleaner way by overriding
> x86_platform.set_wallclock only in the non-init-domain case?

Yes, that makes sense.  John, don't apply this one yet, thanks.

David

Re: [Xen-devel] [PATCH 2/2] x86/xen: sync the wallclock when the system time changes

[John Stultz]

On 05/29/2013 02:37 AM, David Vrabel wrote:
> On 29/05/13 08:47, Jan Beulich wrote:
>>>>> On 28.05.13 at 20:22, David Vrabel <david.vrabel@citrix.com> wrote:
>>> @@ -199,28 +201,59 @@ static void xen_get_wallclock(struct timespec *now)
>>>   
>>>   static int xen_set_wallclock(const struct timespec *now)
>>>   {
>>> -	struct xen_platform_op op;
>>> -	int ret;
>>> -
>>>   	/* do nothing for domU */
>>>   	if (!xen_initial_domain())
>>>   		return -1;
>>>   
>>> -	/* Set the Xen wallclock. */
>>> +	/* Set the hardware RTC. */
>>> +	return mach_set_rtc_mmss(now);
>>> +
>>> +}
>> Can't you achieve the same effect in a cleaner way by overriding
>> x86_platform.set_wallclock only in the non-init-domain case?
> Yes, that makes sense.  John, don't apply this one yet, thanks.
Ok. Zapped from my tree.

thanks
-john