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Re: [Xen-devel] Could you please answer some questions regarding Solaris PVHVM pvclock support.

于 2014/1/16 20:17, Stefano Stabellini 写道:
. 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
    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 = "">
    do {
        pre_version = xen_fetch_vcpu_tinfo(&dst, src);
        now = dst.system_time + get_nsec_offset(&dst);
    } 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;
    } while (!atomic_cmpset_64(&last_time, last, now));


    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?


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