Re: [Xen-devel] when timer go back in dom0 save and restore or migrate, PV domain hung

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

The problem hasn't been fully explained, but I can say that PV guests expect system time to jump across s/r and deal with that. For example, Linux doesn't use Xen system time internally, but uses its progress to periodically update jiffies, which does not warp across s/r.

We have had problems corrupting wc_sec/wc_nsec in xc_domain_restore.c, but that was fixed some time ago.

 -- Keir

On 26/11/08 14:00, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

This is not a s/r or lm specific issue. For example, system time can be changed even when pv guest is running. Your patch only hacks restore point once, and wc_sec can still be changed later when system time is changed on-the-fly again.

IIRC, pv guest can catch up wall clock change in timer interrupt, and time_resume will sync internal processed system time with new system time after restored. But I'm not sure whether it's enough. Actually the more interesting is the uptime difference. For example, timer with expiration calculated on previous system time may wait nearly infinite if uptime among two boxes vary a lot. But I think such issue should have been considered already, e.g. some user tool assistance. I think Keir can comment better here.

BTW, do you happen to know what exactly dom0 hangs on? In some busy loop to catch up time, or long delay to some critical timer expiration?

Thanks,
Kevin

 

---

From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx  [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of James  Song
Sent: Tuesday, November 25, 2008 4:02 PM
To:  xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] when timer go  back in dom0 save and restore or migrate, PV domain hung

 
Hi,
   I find PV domin hung, When we take those steps  
         1, save PV domain  
         2, change system time of  PV domain back
         3, restore  a PV domain
        or  
         1, migrate a PV domain  from Machine A to Machine B
         2, the system  time of Machine B is slower than Machine A.
   the problem is  wc_sec will be change when system-time chanaged in dom0 or restore in a  slower-system-time machine, but when restoring, xen don't restore the wc_sec  of share_info from xenstore and use native one. So guest os will hang.  
this patch will work for this issue.

 Thanks
 -- Song  Wei

diff -r a5ed0dbc829f tools/libxc/xc_domain_restore.c
---  a/tools/libxc/xc_domain_restore.c    Tue Nov 18 14:34:14 2008  +0800
+++ b/tools/libxc/xc_domain_restore.c    Fri Nov 21  17:34:15 2008 +0800
@@ -328,6 +328,16  @@
 
     /* For info only  */
     nr_pfns = 0;
+     //jsong@xxxxxxxxxx, james song
+    memset(&domctl, 0,  sizeof(domctl));
+    domctl.domain =  dom;
+    domctl.cmd    =  XEN_DOMCTL_restoredomain;
+    frc = do_domctl(xc_handle,  &domctl);
+    if ( frc != 0 )
+     {
+             ERROR("Unable to set flag of  restore.");
+             goto out;
+    }
 
     if (  read_exact(io_fd, &p2m_size, sizeof(unsigned long))  )
     {
@@ -1120,6 +1130,8  @@
 
     /* restore saved vcpu_info and arch  specific info */
     MEMCPY_FIELD(new_shared_info,  old_shared_info, vcpu_info);
+     MEMCPY_FIELD(new_shared_info, old_shared_info,  wc_nsec);
+    MEMCPY_FIELD(new_shared_info,  old_shared_info, wc_sec);
      MEMCPY_FIELD(new_shared_info, old_shared_info,  arch);
 
     /* clear any pending events and  the selector */
diff -r a5ed0dbc829f xen/arch/x86/time.c
---  a/xen/arch/x86/time.c    Tue Nov 18 14:34:14 2008 +0800
+++  b/xen/arch/x86/time.c    Fri Nov 21 17:34:15 2008 +0800
@@  -689,7 +689,6 @@
      wmb();
     (*version)++;
 }
-
 void  update_vcpu_system_time(struct vcpu *v)
 {
      struct cpu_time       *t;
@@ -703,7 +702,6  @@
 
     if ( u->tsc_timestamp ==  t->local_tsc_stamp )
          return;
-
      version_update_begin(&u->version);
 
      u->tsc_timestamp     = t->local_tsc_stamp;
@@  -713,14 +711,19 @@
 
      version_update_end(&u->version);
 }
-
 void  update_domain_wallclock_time(struct domain  *d)
 {
      spin_lock(&wc_lock);
+    if(d->after_restore  )
+    {
+         d->after_restore = 0;
+        goto  out; //jsong@xxxxxxxxxx
+    }
      version_update_begin(&shared_info(d,  wc_version));
     shared_info(d, wc_sec)  =  wc_sec + d->time_offset_seconds;
     shared_info(d,  wc_nsec) = wc_nsec;
      version_update_end(&shared_info(d,  wc_version));
+out:
      spin_unlock(&wc_lock);
 }
 
@@ -751,7 +754,6  @@
     u64 x;
     u32 y,  _wc_sec, _wc_nsec;
     struct domain  *d;
-
     x = (secs * 1000000000ULL) + (u64)nsecs -  system_time_base;
     y = do_div(x,  1000000000);
 
@@ -1050,7 +1052,6 @@
 struct tm  wallclock_time(void)
 {
     uint64_t  seconds;
-
     if ( !wc_sec  )
         return (struct tm) { 0  };
 
diff -r a5ed0dbc829f xen/common/domctl.c
---  a/xen/common/domctl.c    Tue Nov 18 14:34:14 2008 +0800
+++  b/xen/common/domctl.c    Fri Nov 21 17:34:15 2008 +0800
@@  -24,7 +24,6 @@
 #include <asm/current.h>
 #include  <public/domctl.h>
 #include  <xsm/xsm.h>
-
 extern long  arch_do_domctl(
     struct xen_domctl *op,  XEN_GUEST_HANDLE(xen_domctl_t) u_domctl);
 
@@ -315,6 +314,16  @@
         ret =  0;
     }
      break;
+   case XEN_DOMCTL_restoredomain:
+    {
+        struct domain  *d;
+        if ( (d =  rcu_lock_domain_by_id(op->domain)) == NULL  )
+             break;
+         
+        d->after_restore =  1;
+         rcu_unlock_domain(d);
+         break;
+   }
 
     case  XEN_DOMCTL_createdomain:
     {
diff -r a5ed0dbc829f  xen/include/public/domctl.h
---  a/xen/include/public/domctl.h    Tue Nov 18 14:34:14 2008  +0800
+++ b/xen/include/public/domctl.h    Fri Nov 21  17:34:15 2008 +0800
@@ -61,6 +61,7 @@
 #define  XEN_DOMCTL_destroydomain      2
 #define  XEN_DOMCTL_pausedomain         3
 #define XEN_DOMCTL_unpausedomain       4
+#define XEN_DOMCTL_restoredomain       51
 #define XEN_DOMCTL_resumedomain       27
 
 #define  XEN_DOMCTL_getdomaininfo      5
diff -r  a5ed0dbc829f xen/include/xen/sched.h
---  a/xen/include/xen/sched.h    Tue Nov 18 14:34:14 2008  +0800
+++ b/xen/include/xen/sched.h    Fri Nov 21 17:34:15  2008 +0800
@@ -231,6 +231,7 @@
      * cause a  deadlock. Acquirers don't spin waiting; they  preempt.
      */
      spinlock_t hypercall_deadlock_mutex;
+    int after_restore;  //jsong@xxxxxxxxxx
 };
 
 struct  domain_setup_info
---------------------------------------------------------------------------------------------
 Thanks
--Song  wei

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