Hi Leo,

Two points here - both related.

On Tue, 22 Oct 2019 at 06:10, Leo Yan [off-list ref] wrote:

Hi Mathieu,

On Fri, Oct 11, 2019 at 02:16:06PM -0600, Mathieu Poirier wrote:

quoted

On Sat, Oct 05, 2019 at 05:16:09PM +0800, Leo Yan wrote:

quoted

If the u64 variable 'offset' is a negative integer, comparison it with
bigger than zero is always going to be true because it is unsigned.
Fix this by using s64 type for variable 'offset'.

Signed-off-by: Leo Yan <redacted>
---
 tools/perf/util/cs-etm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 4ba0f871f086..4bc2d9709d4f 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c

@@ -940,7 +940,7 @@ u64 cs_etm__last_executed_instr(const struct cs_etm_packet *packet)
 static inline u64 cs_etm__instr_addr(struct cs_etm_queue *etmq,
                                 u64 trace_chan_id,
                                 const struct cs_etm_packet *packet,
-                                u64 offset)
+                                s64 offset)

Issue 1:

OK - it appears that cs_etm__instr_addr() is supposed to be returning
the address within the current trace sample of the instruction related
to offset.
For T32 - then if offset < 0, packet->start_addr is returned - not
good but at least within the current trace range
For A32/A64 - if offset < 0 then an address _before_
packet->start_addr is returned - clearly wrong and possibly a
completely invalid address that was never actually traced.

quoted

In Suzuki's reply there was two choices, 1) move the while(offset > 0) to
while (offset) or change the type of @offset to an s64.  Here we know offset
can't be negative because of the
        tidq->period_instructions >= etm->instructions_sample_period

in function cs_etm__sample().  As such I think option #1 is the right way to
deal with this rather than changing the type of the variable.

I took sometime to use formulas to prove that 'offset' is possible to
be a negative value :)

Just paste the updated commit log at here for review:

  Pi: period_instructions
  Ie: instrs_executed
  Io: instrs_over
  Ip: instructions_sample_period

  Pi' = Pi + Ie   -> New period_instructions equals to the old
                     period_instructions + instrs_executed
  Io  = Pi' - Ip  -> period_instructions - instructions_sample_period

  offset = Ie - Io - 1
         = Ie - (Pi' - Ip) -1
         = Ie - (Pi + Ie - Ip) -1
         = Ip - Pi - 1

In theory, if Ip (instructions_sample_period) is small enough and Pi
(period_instructions) is bigger than Ip, then it will lead to the
negative value for 'offset'.

So let's see below command:

  perf inject --itrace=i1il128 -i perf.data -o perf.data.new

With this command, 'offset' is very easily to be a negative value when
handling packets; this is because if use the inject option 'i1', then
instructions_sample_period equals to 1; so:

  offset = 1 - Pi - 1
         = -Pi

Any Pi bigger than zero leads 'offset' to a negative value.

Thanks,
Leo Yan

Issue 2:

Assuming I have understood the logic of this code correctly - there is
an issue were sample_period < period_instructions as you say -
but I believe the problem is in the logic of the sampling function itself.

Suppose we have a sample_period of 4.

Now on an initial pass through the function, period_instructions must
be 0. (i.e. none left over from the previous pass.)
Suppose also that the number of instructions executed in this sample
is 10 - thus updating period_instructions.
Therefore:
instr_over = 10 - 4 -> 6
offset = 10 - 6 - 1 -> 3.
We therefore call cs_etm_instr_addr to find the address an offset of 3
instructions from the start of the trace sample and synthesize the
sample.
After this we set period_instructions to the instr_over value of 6.

Next pass, assume 10 instructions in the trace sample again.
period_instructions = 6 + 10 -> 16
instr_over = 16 - 4 -> 12
offset = 10 - 12 - 1 -> -3  - the negative value your formulae predict.

This implies that the sample we want is actually in the previous trace
packet - which I believe is in fact the case - as explained below.

My reading of the code is that cs_etm__sample() is called once per
trace range packet extracted from the decoder - and a trace range
packet represents N instructions_executed.
Further I am assuming that instructions_sample_period represents the
desired periodicity of the instruction samples - i.e. 1 sample every
instructions_sample_period number of instructions.

Thus my conclusion here is that where M = instructions_executed +
period_instructions, the function should generate quotient ( M /
instructions_sample_period ) samples and set period_instructions to M
mod instructions_sample_period on exit, ensuring period_instructions
is never larger than the sample_period.

i.e. loop to generate multiple samples until instr_over and therefore
the output value of period_instructions is less than the value of
instructions_sample_period - for the example above, with 10
instructions and a periodicity of 4, we generate 2 samples with a
remainder of 2 instructions carried forwards.

In short leave offset as unsigned and fix the logic of the
cs_etm__sample() function.

Regards

Mike

quoted

quoted

 {
    if (packet->isa == CS_ETM_ISA_T32) {
            u64 addr = packet->start_addr;

@@ -1372,7 +1372,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
             * sample is reported as though instruction has just been
             * executed, but PC has not advanced to next instruction)
             */
-           u64 offset = (instrs_executed - instrs_over - 1);
+           s64 offset = (instrs_executed - instrs_over - 1);
            u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
                                          tidq->packet, offset);

--

2.17.1

--
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK

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