Re: Re: Re: [PATCH V2 1/1] kvm/vmx: Add a tracepoint write_tsc_offset

[Marcelo Tosatti <mtosatti@redhat.com>]

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?