Re: when timer go back in dom0 save and restore or migrate, PV domain hung

[Keir Fraser <keir.fraser@eu.citrix.com>]

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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@intel.com> 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@lists.xensource.com
>> [mailto:xen-devel-bounces@lists.xensource.com] On Behalf Of James  Song
>> Sent: Tuesday, November 25, 2008 4:02 PM
>> To:  xen-devel@lists.xensource.com
>> 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@novell.com, 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@novell.com
>> +    }
>>       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@novell.com
>>  };
>>  
>>  struct  domain_setup_info
>> -----------------------------------------------------------------------------
>> ----------------
>>  Thanks
>> --Song  wei
>> 
> 



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