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

On 05/14/2013 11:00 AM, David Vrabel wrote:
On 14/05/13 18:24, John Stultz wrote:
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).

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);
Sorry, just noticed one more thing while applying this. Do all Xen
systems run on hardware that has the conventional CMOS clock?

What happens if the bare-metal needs to use efi_set_rtc() or
vrtc_set_mmss() ?
Ug. I hadn't considered that.  I'll have think some more on this.

Ok. I'll look forward to a future revision then and not queue this one.

Just FYI, here's what I rewrote the commit message to:

    x86/xen: Sync the CMOS RTC as well as the Xen wallclock

    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.

I feel its a bit more clear, but feel free to tweak it as you see fit.


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