Re: [PATCH] perf scripts python arm-cs-trace-disasm.py: Skip disasm if address continuity is broken
From: James Clark
Date: Fri Aug 09 2024 - 11:20:09 EST
On 09/08/2024 3:13 pm, Mike Leach wrote:
Hi James
On Thu, 8 Aug 2024 at 10:32, James Clark <james.clark@xxxxxxxxxx> wrote:
On 07/08/2024 5:48 pm, Leo Yan wrote:
Hi all,
On 8/7/2024 3:53 PM, James Clark wrote:
A minor suggestion: if the discussion is too long, please delete the
irrelevant message ;)
[...]
--- a/tools/perf/scripts/python/arm-cs-trace-disasm.py
+++ b/tools/perf/scripts/python/arm-cs-trace-disasm.py
@@ -257,6 +257,11 @@ def process_event(param_dict):
print("Stop address 0x%x is out of range [ 0x%x .. 0x%x
] for dso %s" % (stop_addr, int(dso_start), int(dso_end), dso))
return
+ if (stop_addr < start_addr):
+ if (options.verbose == True):
+ print("Packet Dropped, Discontinuity detected
[stop_add:0x%x start_addr:0x%x ] for dso %s" % (stop_addr, start_addr,
dso))
+ return
+
I suppose my only concern with this is that it hides real errors and
Perf shouldn't be outputting samples that go backwards. Considering that
fixing this in OpenCSD and Perf has a much wider benefit I think that
should be the ultimate goal. I'm putting this on my todo list for now
(including Steve's merging idea).
In the perf's util/cs-etm.c file, it handles DISCONTINUITY with:
case CS_ETM_DISCONTINUITY:
/*
* The trace is discontinuous, if the previous packet is
* instruction packet, set flag PERF_IP_FLAG_TRACE_END
* for previous packet.
*/
if (prev_packet->sample_type == CS_ETM_RANGE)
prev_packet->flags |= PERF_IP_FLAG_BRANCH |
PERF_IP_FLAG_TRACE_END;
I am wandering if OpenCSD has passed the correct info so Perf decoder can
detect the discontinuity. If yes, then the flag 'PERF_IP_FLAG_TRACE_END' will
be set (it is a general flag in branch sample), then we can consider use it in
the python script to handle discontinuous data.
No OpenCSD isn't passing the correct info here. Higher up in the thread
I suggested an OpenCSD patch that makes it detect the error earlier and
fixes the issue. It also needs to output a discontinuity when the
address goes backwards. So two fixes and then the script works without
modifications.
Which address is going backwards here? - OpenCSD generates trace
ranges only by walking forwards from the last known address till it
hits a branch. Unless this wraps round 0x000000 this will never result
in a backwards address as far as I can see.
Do you have an example dump with OpenCSD outputting a range packet
with backwards addresses?
Mike
The example I have I think is something like this:
1. Start address / trace on
2. E
3. Output range
...
4. Periodic address update
...
5. E
6. Output range
If decode has gone wrong (but undetectably) between steps 1 and 3. Then
the next steps still output a second range based on the last periodic
address received. (I think it might not necessarily have to be a
periodic address but could also be indirect address packet?). Perf
converts the ranges into branch samples by taking the end of the first
range and beginning of the second range. Then the disassembly script
converts those samples into ranges again by taking the source and
destination of the last two branch samples.
The original issue that Ganapat saw was that the periodic address causes
OpenCSD to put the source address of the second range somewhere before
the first one, even though it didn't output a branch or discontinuity
that would explain how it got there.
But yes you're right the ranges themselves always go forwards from the
point of view of their own start and end addresses.
I thought it might be possible for OpenCSD to check against the last
range output? Although I wasn't sure if maybe it's actually valid to do
a backwards jump like that without the trace on/off packets with address
filtering or something?
The root cause is still the incorrect image, but I think this check
along with the other direct branch check should make it pretty difficult
for people to make the mistake.