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SubjectRe: [PATCH V2 1/1] kvm/vmx: Add a tracepoint write_tsc_offset
On Wed, Jun 05, 2013 at 09:23:22PM -0300, Marcelo Tosatti wrote:
> On Tue, Jun 04, 2013 at 05:36:19PM +0900, Yoshihiro YUNOMAE wrote:
> > Add a tracepoint write_tsc_offset for tracing TSC offset change.
> > We want to merge ftrace's trace data of guest OSs and the host OS using
> > TSC for timestamp in chronological order. We need "TSC offset" values for
> > each guest when merge those because the TSC value on a guest is always the
> > host TSC plus guest's TSC offset. If we get the TSC offset values, we can
> > calculate the host TSC value for each guest events from the TSC offset and
> > the event TSC value. The host TSC values of the guest events are used when we
> > want to merge trace data of guests and the host in chronological order.
> > (Note: the trace_clock of both the host and the guest must be set x86-tsc in
> > this case)
> >
> > TSC offset is stored in the VMCS by vmx_write_tsc_offset() or
> > vmx_adjust_tsc_offset(). KVM executes the former function when a guest boots.
> > The latter function is executed when kvm clock is updated. Only host can read
> > TSC offset value from VMCS, so a host needs to output TSC offset value
> > when TSC offset is changed.
> >
> > Since the TSC offset is not often changed, it could be overwritten by other
> > frequent events while tracing. To avoid that, I recommend to use a special
> > instance for getting this event:
> >
> > 1. set a instance before booting a guest
> > # cd /sys/kernel/debug/tracing/instances
> > # mkdir tsc_offset
> > # cd tsc_offset
> > # echo x86-tsc > trace_clock
> > # echo 1 > events/kvm/kvm_write_tsc_offset/enable
> >
> > 2. boot a guest
> >
> > Signed-off-by: Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>
> > Cc: Marcelo Tosatti <mtosatti@redhat.com>
> > Cc: Gleb Natapov <gleb@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > ---
> > arch/x86/kvm/trace.h | 18 ++++++++++++++++++
> > arch/x86/kvm/vmx.c | 3 +++
> > arch/x86/kvm/x86.c | 1 +
> > 3 files changed, 22 insertions(+)
> >
> > diff --git a/arch/x86/kvm/trace.h b/arch/x86/kvm/trace.h
> > index fe5e00e..9c22e39 100644
> > --- a/arch/x86/kvm/trace.h
> > +++ b/arch/x86/kvm/trace.h
> > @@ -815,6 +815,24 @@ TRACE_EVENT(kvm_track_tsc,
> > __print_symbolic(__entry->host_clock, host_clocks))
> > );
> >
> > +TRACE_EVENT(kvm_write_tsc_offset,
> > + TP_PROTO(__u64 previous_tsc_offset, __u64 next_tsc_offset),
> > + TP_ARGS(previous_tsc_offset, next_tsc_offset),
> > +
> > + TP_STRUCT__entry(
> > + __field( __u64, previous_tsc_offset )
> > + __field( __u64, next_tsc_offset )
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->previous_tsc_offset = previous_tsc_offset;
> > + __entry->next_tsc_offset = next_tsc_offset;
> > + ),
> > +
> > + TP_printk("previous=%llu next=%llu",
> > + __entry->previous_tsc_offset, __entry->next_tsc_offset)
> > +);
> > +
>
> Yoshihiro YUNOMAE,
>
> 1) Why is previous_tsc_offset necessary?
>
> 2) The TSC offset traces should include vcpu number, so that its
> possible to correlate traces of SMP guests (the tool should use
> the individual vcpu tsc offsets when converting guests trace).
>
Why PID is not enough? No other trace, except kvm_entry, outputs vcpu id.

> 3) Please add traces for svm.c.

--
Gleb.


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