Re: Re: Re: Re: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSCoffset

[Yoshihiro YUNOMAE]

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?

Thanks,

--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@xxxxxxxxxxx


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