Re: [PATCH bpf-next] bpf: introduce helper bpf_raw_read_cpu_clock

[Peter Zijlstra]

On Wed, Oct 06, 2021 at 02:37:09PM -0700, Song Liu wrote:
> On Wed, Oct 6, 2021 at 10:52 AM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> >
> >
> >
> > Add bpf_raw_read_cpu_clock helper, to read architecture specific
> > CPU clock. In x86's case, this is the TSC.
> >
> > This is necessary to synchronize bpf traces from host and guest bpf-programs
> > (after subtracting guest tsc-offset from guest timestamps).
> 
> Trying to understand the use case. So in a host-guest scenario,
> bpf_ktime_get_ns()
> will return different values in host and guest, but rdtsc() will give
> the same value.
> Is this correct?

No, it will not. Also, please explain if any of this stands a chance of
working for anything other than x86. Or even on x86 in the face of
guest migration.

Also, please explain, again, what's wrong with dumping snapshots of
CLOCK_MONOTONIC{,_RAW} from host and guest and correlating time that
way?

And also explain why BPF needs to do this differently than all the other
tracers.