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    SubjectRe: [PATCH] BUG in pv_clock when overflow condition is detected
    On 02/16/2012 03:03 PM, Avi Kivity wrote:
    > On 02/15/2012 07:18 PM, Igor Mammedov wrote:
    >>> On 02/15/2012 01:23 PM, Igor Mammedov wrote:
    >>>>>> static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time
    >>>>>> *shadow)
    >>>>>> {
    >>>>>> - u64 delta = native_read_tsc() - shadow->tsc_timestamp;
    >>>>>> + u64 delta;
    >>>>>> + u64 tsc = native_read_tsc();
    >>>>>> + BUG_ON(tsc< shadow->tsc_timestamp);
    >>>>>> + delta = tsc - shadow->tsc_timestamp;
    >>>>>> return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
    >>>>>> shadow->tsc_shift);
    >>>>>
    >>>>> Maybe a WARN_ON_ONCE()? Otherwise a relatively minor hypervisor
    >>>>> bug can
    >>>>> kill the guest.
    >>>>
    >>>>
    >>>> An attempt to print from this place is not perfect since it often
    >>>> leads
    >>>> to recursive calling to this very function and it hang there
    >>>> anyway.
    >>>> But if you insist I'll re-post it with WARN_ON_ONCE,
    >>>> It won't make much difference because guest will hang/stall due
    >>>> overflow
    >>>> anyway.
    >>>
    >>> Won't a BUG_ON() also result in a printk?
    >> Yes, it will. But stack will still keep failure point and poking
    >> with crash/gdb at core will always show where it's BUGged.
    >>
    >> In case it manages to print dump somehow (saw it couple times from ~
    >> 30 test cycles), logs from console or from kernel message buffer
    >> (again poking with gdb) will show where it was called from.
    >>
    >> If WARN* is used, it will still totaly screwup clock and
    >> "last value" and system will become unusable, requiring looking with
    >> gdb/crash at the core any way.
    >>
    >> So I've just used more stable failure point that will leave trace
    >> everywhere it manages (maybe in console log, but for sure in stack)
    >> in case of WARN it might leave trace on console or not and probably
    >> won't reflect failure point in stack either leaving only kernel
    >> message buffer for clue.
    >>
    >
    > Makes sense. But do get an ack from the Xen people to ensure this
    > doesn't break for them.
    >
    Konrad, Ian

    Could you please review patch form point of view of xen?
    Whole thread could be found here https://lkml.org/lkml/2012/2/13/286

    --
    Thanks,
    Igor


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