Re: [PATCH v2 02/12] simpletrace: Annotate magic constants from QEMU code

[Stefan Hajnoczi]

On Tue, May 02, 2023 at 11:23:29AM +0200, Mads Ynddal wrote:
> From: Mads Ynddal <m.ynddal@samsung.com>
> 
> It wasn't clear where the constants and structs came from, so I added
> comments to help.
> 
> Signed-off-by: Mads Ynddal <m.ynddal@samsung.com>
> ---
>  scripts/simpletrace.py | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/scripts/simpletrace.py b/scripts/simpletrace.py
> index 9211caaec1..7ba805443d 100755
> --- a/scripts/simpletrace.py
> +++ b/scripts/simpletrace.py
> @@ -15,15 +15,15 @@
>  from tracetool import read_events, Event
>  from tracetool.backend.simple import is_string
>  
> -header_event_id = 0xffffffffffffffff
> -header_magic    = 0xf2b177cb0aa429b4
> -dropped_event_id = 0xfffffffffffffffe
> +header_event_id = 0xffffffffffffffff # trace/simple.c::HEADER_EVENT_ID
> +header_magic    = 0xf2b177cb0aa429b4 # trace/simple.c::HEADER_MAGIC
> +dropped_event_id = 0xfffffffffffffffe # trace/simple.c::DROPPED_EVENT_ID
>  
> -record_type_mapping = 0
> -record_type_event = 1
> +record_type_mapping = 0 # trace/simple.c::TRACE_RECORD_TYPE_MAPPING
> +record_type_event = 1 # trace/simple.c::TRACE_RECORD_TYPE_EVENT
>  
> -log_header_fmt = '=QQQ'
> -rec_header_fmt = '=QQII'
> +log_header_fmt = '=QQQ' # trace/simple.c::TraceLogHeader
> +rec_header_fmt = '=QQII' # trace/simple.c::TraceRecord

From my reply to v1 of this patch series:

This is fragile since this information will be outdated if the C source
code changes (e.g. renaming files or variables).

Instead I would add the following comment:

  # This is the binary format that the QEMU "simple" trace backend
  # emits. There is no specification documentation because the format is
  # not guaranteed to be stable. Trace files must be parsed with the
  # same trace-events-all file and the same simpletrace.py file that
  # QEMU was built with.

I hope that clarifies the scope of the binary format and someone wishing
to look into the format would then know to look at the "simple" trace
backend.

Stefan

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