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Re: [Xen-devel] [PATCH 3/3] x86/xen: sync the CMOS RTC as well as the Xen wallclock

On 05/13/2013 10:56 AM, David Vrabel wrote:
From: David Vrabel <david.vrabel@xxxxxxxxxx>

If NTP is used in dom0 and it is synchronized to its clock source,
then the kernel will periodically synchronize the Xen wallclock with
the system time.  Updates to the Xen wallclock do not persist across
reboots, so also synchronize the CMOS RTC (as on bare metal).

Sorry again, not getting this one either.

So normally in this case we're using the Xen wallclock as the underlying source for the persistent_clock here, my understanding is we use this instead of the standard cmos, because we get benefits of using the hypervisor's sense of time instead of the bare hardware, and allows for virtualization of the persistent clock.

But the problem is that even if Dom0 tries to set the xen persistent clock, it doesn't actually update anything in the underlying hardware? So here you instead try to sync the underlying hardware cmos from the same Xen dom0 environment?

Honestly, it seems a little strange to me. If you're running as dom0, why does HYPERVISOR_dom0_op() not cause the hypervisor to set the cmos its virtualizing? This seems to mess with the proper virtualization layering.

I'd appreciate some more details and maybe some feedback from other xen folks here to make sure this really makes sense.


Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
  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 = {

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