On 31/05/13 01:30, John Stultz wrote:On 05/30/2013 07:25 AM, David Vrabel wrote:This is correct. I'll add an explanatory paragraph about the XenFrom: David Vrabel <david.vrabel@xxxxxxxxxx>Ok.. So this is maybe starting to make a little more sense.
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.
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.
wallclock to the changelog.
Using the notifier allows step changes to be noticed immediately, usingUse 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?
just a timer would leave a window after any step change where the Xen
wallclock is wrong.
Ideally, I would like a notification of step changes and a long period
timer (to correct for drift). Can you think of a good way to do this?
This would probably be better done as:--- a/arch/x86/xen/time.cThis comment needs some improvement. It doesn't explain why Xen needs to
+++ b/arch/x86/xen/time.c
@@ -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.
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.
if abs(current_wallclock - current_kernel_time) > threshold)
update_wallclock();
i.e., we're correcting the wallclock if it is wrong.
The notifier chain is called from timekeeping_update() which is called+ */You don't seem to be holding the timekeeping lock here, so why are you
+
+ now = __current_kernel_time();
calling the internal __ prefixed current_kernel_time() accessor?
in a write_seqcount_begin/end(&timekeeper_seq) block.
This is to handle step changes that go backwards.+Not sure I understand why you're bothering with the last value? Aren't
+ if (timespec_compare(&now, &last) > 0
you just wanting to trigger when now is greater then next?