On Fri, Nov 30, 2012 at 10:36:43AM +0900, Yoshihiro YUNOMAE wrote: > Hi Marcelo, > > >>>Can you please write a succint but complete description of the method > >>>so it can be verified? > >> > >>Sure. > >> > >>- Prerequisite > >>1. the host TSC is synchronized and stable. > >>2. kvm_write_tsc_offset events include previous and next TSC offset > >> values. > >>3. Every event's trace_clock is TSC. > >> > >>- Assumption for the sake of ease > >>1. One VCPU > >>2. The guest executes no write_tsc or write_tsc only once. > >> > >>- Configuration > >>1. On the host, kvm_exit/entry events are recorded in the buffer A and > >> kvm_write_tsc_offset events are recorded in the buffer B. > >>2. Boot a guest > >> > >>- Sort method > >>1. > >> Confirm which the kvm_write_tsc_offset events are recorded except for > >>boot. Note that a vcpu thread writes TSC offset when boot as an initial > >>operation. > >> > >>1-1. > >> If not recorded, it means that the guest did not execute write_tsc. > >>So, we convert the guest TSC to the host TSC using TSC offset of boot. > >>=> END > >> > >>1-2. > >> If recorded, it means that the guest executed write_tsc. > >>So, we use new kvm_write_tsc_offset event information. > >> > >>2. > >>We convert the host TSC(Th) of the kvm_write_tsc_offset event to > >>the guest TSC(Tg) using previous_tsc_offset value: > >> Tg = Th + previous_tsc_offset > >> > >>3. > >>To search the point where the guest executed write_tsc, we find "n" > >>which satisfies the condition Tn < Tg < Tn+1 or Tn+1 < Tn < Tg from > >>older events of the guest. > >>The former condition means trace data of > >>the guest were recorded monotonically. On the other hand, the latter > >>condition means trace data of the guest moved backward. > >>4. > >>We convert the guest TSC of trace data to the host TSC using > >>previous_tsc_offset value before "n" and using next_tsc_offset value > >>after "n+1". > >>=> END > >> > >>- Note > >>We assumed one vcpu and no write_tsc or write_tsc only once for the > >>sake of ease. For other conditions, we will use similar method. > >> > >>Thanks, > > > >There is no certainty. Consider the following information available > > > >guest trace host trace > > 100: guest_tsc_write (tsc_offset=-100 => guest_tsc = 0) > > 104: guest_tsc_write (tsc_offset=-104 => guest_tsc = 0) > > 108: guest_tsc_write (tsc_offset=-108 => guest_tsc = 0) > >1: eventA > >2: eventB > >3: eventC > >1: eventD > >2: eventE > >3: eventF > > > >How can you tell which tsc_offset to use for eventD ? It could be either > >-104 or -108. The notion of "next_tsc_offset" is subject to such > >issue. > > > >I suppose its fine to restrict the interface as follows: the tool will > >accept one trace of events with monotonic timestamps and the user is > >responsible for correlating that to a host trace. > > OK, I'll add the restriction, which trace data must have monotonic > timestamps to use this feature. I think guests seldom will execute > write_tsc, so in many cases, timestamps will be monotonically recorded > in trace data. > > >That is, you can't feed distinct instances of guest kernel trace. > > I'm not clear for "distinct instances". Is this about SMP or multiple > guests? Would you explain about this? Distinct boot instances. If the guest reboots TSC can be written to. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
