Re: [PATCH 2/3] coresight: Fail to open with return stacks if they are unavailable

[Mike Leach <mike.leach@linaro.org>]

Hi James

On Fri, 17 Dec 2021 at 09:41, James Clark <james.clark@arm.com> wrote:
>
>
>
> On 13/12/2021 09:48, Mike Leach wrote:
> > Hi James,
> >
> > A couple of points - relating mainly to docs:
> >
>
> Hi Mike,
>
> Thanks for the feedback. I was in the process of adding some docs and
> ran into this https://lkml.org/lkml/2021/12/16/1087 so I went to fix
> that first. Now I will add some more details and resubmit.
>
> > 1. Activating branch broadcast overrides any setting of return stack.
> > As a minimum there needs to be a documentation update to reflect this
> > -Setting both options is not prohibited in hardware - and in the case
> > where we can use branch broadcast over a range both are then relevant.
> >
>
> Do you mean that if branch broadcast and return stacks are both requested,
> but branch broadcast is limited to a range, return stacks will only be
> available outside that branch broadcast range? But if branch broadcast is
> enabled for all ranges there will be no return stacks at all?
>

Correct - you may want to branch broadcast over a small ranges, but
otherwise use return stack to decrease the amount of trace in other
areas.
Once the complex config sets are accepted upstream then this set-up
will be possible by writing a config to do this.

> > 2. A documented note to reflect that choosing this option will result
> > in a significant increase in the amount of trace generated - possible
> > danger of overflows, or less actual instructions covered. In addition
> > perhaps documents could reflect the intended use-case for this option,
> > given the disadvantages.
> >
>
> Will do.
>
> > 3. Has this been tested in a situation where it will be of use?
> > Testing against static code images will show the same decoded trace
> > output as not using branch broadcast. (although the packet dumps will
> > show additional output)>
> > Given a primary use is for situations where code is patched or
> > dynamically altered at runtime - then this can affect the full decode
> > output. If the code is being patched to only alter the branch
> > addresses then decode should work against static images.
> > If, however, we are tracing code that adds in new branches, on top of
> > NOPs for example, then the decoding against the original static image
> > will be wrong, as the image will have the NOPs, rather than the branch
> > instructions so the apparent location of E atoms will be in a
> > different position to the actual code. Is there anything in perf that
> > will ensure that the patched code is presented to the decoder?
> >
> > If there are potential decode issues - these too need documenting.
> >
>
> I'm not sure this should be a blocking issue for this set. Branch broadcast
> could already be enabled by setting the mode via sysfs. And the perf decode
> part isn't necessarily a step in the workflow, maybe someone wants to gather
> data for another tool.
>

Someone could set this in sysfs - when collecting data via sysfs.  In
this case they would not be using perf for decode anyway as you say.

What you have added here is a new method for perf to set this feature
and perf always starts off with a clean configuration - then adds
according to command line options.
Therefore this will be the first time anyone will have been able to
set this for a perf session.

It there are potential limitations, then we need to be clear about
them - then users of perf, and other hypothetical tools can make a
good choice about if this option is appropriate - and we avoid mailing
list complaints (or at least can point to the docs) if users find
issues that they were warned about!

> I will do some testing after this change though, but I imagine we would have
> had issues reported it it wasn't working already which lowers the priority.
>

really depends on what was tested! If it is being used over static
code then the subsequent decode will be fine,

Regards

Mike

> James
>
> > Other than the documents and testing,  I cannot see any issues with
> > this patch set in terms of setting and enabling the option.
> >
> > Regards
> >
> > Mike
> >
> >
> > On Fri, 10 Dec 2021 at 17:22, Mathieu Poirier
> > <mathieu.poirier@linaro.org> wrote:
> >>
> >> Hi James,
> >>
> >> On Thu, Dec 09, 2021 at 11:13:55AM +0000, James Clark wrote:
> >>>
> >>>
> >>> On 09/12/2021 11:00, Suzuki K Poulose wrote:
> >>>> On 08/12/2021 16:09, James Clark wrote:
> >>>>> Maintain consistency with the other options by failing to open when they
> >>>>> aren't supported. For example ETM_OPT_TS, ETM_OPT_CTXTID2 and the newly
> >>>>> added ETM_OPT_BRANCH_BROADCAST all return with -EINVAL if they are
> >>>>> requested but not supported by hardware.
> >>>>>
> >>>>> The consequence of not doing this is that the user may not be
> >>>>> aware that they are not enabling the feature as it is silently disabled.
> >>>>>
> >>>>> Signed-off-by: James Clark <james.clark@arm.com>
> >>>>> ---
> >>>>>   drivers/hwtracing/coresight/coresight-etm4x-core.c | 13 +++++++++----
> >>>>>   1 file changed, 9 insertions(+), 4 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> >>>>> index d2bafb50c66a..0a9bb943a5e5 100644
> >>>>> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> >>>>> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> >>>>> @@ -674,10 +674,15 @@ static int etm4_parse_event_config(struct coresight_device *csdev,
> >>>>>       }
> >>>>>         /* return stack - enable if selected and supported */
> >>>>> -    if ((attr->config & BIT(ETM_OPT_RETSTK)) && drvdata->retstack)
> >>>>> -        /* bit[12], Return stack enable bit */
> >>>>> -        config->cfg |= BIT(12);
> >>>>> -
> >>>>> +    if (attr->config & BIT(ETM_OPT_RETSTK)) {
> >>>>> +        if (!drvdata->retstack) {
> >>>>> +            ret = -EINVAL;
> >>>>> +            goto out;
> >>>>> +        } else {
> >>>>> +            /* bit[12], Return stack enable bit */
> >>>>> +            config->cfg |= BIT(12);
> >>>>> +        }
> >>>>
> >>>> nit: While at this, please could you change the hard coded value
> >>>> to ETM4_CFG_BIT_RETSTK ?
> >>>>
> >>> I started changing them all because I had trouble searching for bits by name but then
> >>> I thought it would snowball into a bigger change so I undid it.
> >>>
> >>> I think I'll just go and do it now if it's an issue here.
> >>
> >> I can apply this set right away and you send another patch to fix all hard coded
> >> bitfields or you can send another revision with all 4 patches included in it
> >> (bitfields fix plus these 3).  Just let me know what you want to do.  And next
> >> time please add a cover letter.
> >>
> >> Thanks,
> >> Mathieu
> >>
> >>>
> >>>> Otherwise, looks good to me
> >>>>
> >>>> Suzuki
> >
> >
> >



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