Re: Could you please answer some questions regarding Solaris PVHVM pvclock support.

[Qin Li <qin.l.li@oracle.com>]

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? 2014/1/16 20:17, Stefano Stabellini ??:
>> . For a PV-on-HVM guest OS, the "shared_info->vcpu_info->vcpu_time_info"
>> >is already visiable. Does guest OS still need any action to ask
>> >hypervisor to update this piece of memory periodically?
> I don't think you need to ask the hypervisor to update vcpu_time_info
> periodically, what gave you that idea?
Hi Stefano,

Now, I see it's the hypervisor that will update vcpu_time_info, but 
another thing confuse me:
HVM guest has time drift issue because TSC on different vCPU could be 
out-of-sync, especially after domain suspend/resume.
But how pvclock actually fix this issue? Let's see how FreeBSD port 
calculate the system time:

==================
static uint64_t
get_nsec_offset(struct vcpu_time_info *tinfo)
{

     return (scale_delta*(rdtsc() -* tinfo->tsc_timestamp,
         tinfo->tsc_to_system_mul, tinfo->tsc_shift));
}

/**
  * \brief Get the current time, in nanoseconds, since the hypervisor 
booted.
  *
  * \note This function returns the current CPU's idea of this value, unless
  *       it happens to be less than another CPU's previously determined 
value.
  */
static uint64_t
xen_fetch_vcpu_time(void)
{
     struct vcpu_time_info dst;
     struct vcpu_time_info *src;
     uint32_t pre_version;
     uint64_t now;
     volatile uint64_t last;
     struct vcpu_info *vcpu = DPCPU_GET(vcpu_info);

     src = &vcpu->time;

     critical_enter();
     do {
         pre_version = xen_fetch_vcpu_tinfo(&dst, src);
         barrier();
         now = dst.system_time + *get_nsec_offset(&dst);*
         barrier();
     } while (pre_version != src->version);

     /*
      * Enforce a monotonically increasing clock time across all
      * VCPUs.  If our time is too old, use the last time and return.
      * Otherwise, try to update the last time.
      */
     do {
         last = last_time;
         if (last > now) {
             now = last;
             break;
         }
     } while (!atomic_cmpset_64(&last_time, last, now));

     critical_exit();

     return (now);
}
==================================

I guest linux guest will do the same thing, rdtsc() fetch current 
timestamp from current running vCPU, TSC out-of-sync issue is still there.
It seems to me pvclock finally fix the time drift issue just because the 
workaround enforced as above, right?

Thanks,
Michael



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