Re: [PATCH v3 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

From: Peter Zijlstra
Date: Wed Feb 11 2015 - 04:33:25 EST

On Tue, Feb 10, 2015 at 04:22:50PM -0800, Alexei Starovoitov wrote:
> >> not all tools use libtraceevent.
> >> gdb calls perf_event_open directly:
> >>;a=blob;f=gdb/nat/linux-btrace.c
> >> and parses PERF_RECORD_SAMPLE as a binary.
> >> In this case it's branch records, but I think we never said anywhere
> >> that PERF_SAMPLE_IP | PERF_SAMPLE_ADDR should come
> >> in this particular order.
> >
> > What particular order? Note, that's a hardware event, not a software
> > one.
> yes, but gdb assumes that 'u64 ip' precedes, 'u64 addr'
> when attr.sample_type = IP | ADDR whereas this is an
> internal order of 'if' statements inside perf_output_sample()...

This is indeed promised in the data layout description in

There is no other way to find where fields are; perf relies on
predetermined order of fields coupled with the requested field bitmask.

So we promise the order: id, ip, pid, tid, time, addr,.. etc.

So if you request IP and ADDR but none of the other fields, then you
know your sample will start with IP and then contain ADDR.

The traceevent thing has a debug/trace-fs format description of fields
that is supposed to be used.
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