Messages in this thread | | | Date | Mon, 10 Jun 2013 17:28:23 -0300 | From | Marcelo Tosatti <> | Subject | Re: Re: Re: [PATCH V2 1/1] kvm/vmx: Add a tracepoint write_tsc_offset |
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On Mon, Jun 10, 2013 at 07:38:34PM +0300, Gleb Natapov wrote: > > Guest traces contain vcpu number and not pid (because guest is unaware > > of host PID). > > > No, guest trace is just a regular ftrace done inside a guest. It contains > guest's PIDs which is useless for host.
# tracer: nop # # entries-in-buffer/entries-written: 5333/5333 #P:4 # # _-----=> irqs-off # / _----=> need-resched # | / _---=> hardirq/softirq # || / _--=> preempt-depth # ||| / delay # TASK-PID CPU# |||| TIMESTAMP FUNCTION # | | | |||| | |
Traces contain CPU ID.
> I do not know how exactly guest traces are transfered to a host, if > each vcpu buffer is transfered separately host can figure out what > trace entry belong to which vcpu based on what buffer the trace is in. > But the information about what buffer belongs to which vcpu id should > be transfered to a host somehow too. > > > > > However, when we > > > > focus on output data of the write_tsc_offset event, it is difficult to > > > > directly understand contents of the data if vcpu number information is > > > > not included. So, including the information is useful, I think. > > > > > > > How your tool does it now? > > > > It merges guest trace with host trace (by converting the TSC timestamp > > in the guest trace to host TSC using tsc_offset information). > > > I mean how it does it now without vcpu id. The answer is that it works > for only one vcpu now.
Yes.
> > By not recording vcpu ID in the tsc_offset trace, it is necessary to > > supply the tool with PID<->VCPU_id tuples for translation (so its an > > additional step required, and it makes trace merge impossible > > if the information is not available). > The tool needs PID<->VCPU_id tuples to do the merging of any trace > entry. Without that it does not know how to interpret entry timestamps > (which offset to use). Apparently it will get this information from > vmentry trace point. What is so special about tsc_offset tracing that > it needs to contain vcpuid by itself.
If the tsc_offset tracepoint contains vcpu ID, its possible to lookup guest trace entry (which contains CPU ID), and match on that.
Without that, PID<->VCPU_id tuples are necessary. Yes?
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