Re: [PATCH v1 0/7] coresight: etm-perf: Fix pid tracing with VHE

[Leo Yan]

On Mon, Jan 11, 2021 at 11:16:46AM -0700, Mathieu Poirier wrote:
> Hi Leo,
> 
> Suzuki and Mike have pointed out a few things to modify and there was a couple
> of kernel bot warnings to address as well.  As such I will wait for your next
> revision before looking at this set.

Yeah, this is fine for me.

> Thanks,
> Mathieu
> 
> On Sat, Jan 09, 2021 at 03:44:28PM +0800, Leo Yan wrote:
> > This patch series is a following up for the previous version which was
> > delivered by Suzuki [1].  Below gives the background info for why we
> > need this patch series, directly quotes the description in the cover
> > letter of the previous version:
> > 
> > "With the Virtualization Host Extensions, the kernel can run at EL2.
> > In this case the pid is written to CONTEXTIDR_EL2 instead of the
> > CONTEXTIDR_EL1. Thus the normal coresight tracing will be unable
> > to detect the PID of the thread generating the trace by looking
> > at the CONTEXTIDR_EL1. Thus, depending on the kernel EL, we must
> > switch to tracing the correct CONTEXTIDR register.
> > 
> > With VHE, we must set the TRCCONFIGR.VMID and TRCCONFIGR.VMID_OPT
> > to include the CONTEXTIDR_EL2 as the VMID in the trace. This
> > requires the perf tool to detect the changes in the TRCCONFIGR and
> > use the VMID / CID field for the PID. The challenge here is for
> > the perf tool to detect the kernel behavior.
> > 
> > Instead of the previously proposed invasive approaches, this set
> > implements a less intrusive mechanism, by playing with the
> > perf_event.attribute.config bits."
> > 
> > Same as the previous series, this series keeps the same implementation
> > for two introduced format bits:
> > 
> > - contextid_in_vmid -> Is only supported when the VMID tracing
> >   and CONTEXTIDR_EL2 both are supported. When requested the perf
> >   etm4x backend sets (TRCCONFIGR.VMID | TRCCONFIGR.VMID_OPT).
> >   As per ETMv4.4 TRM, when the core supports VHE, the CONTEXTIDR_EL2
> >   tracing is mandatory. (See the field TRCID2.VMIDOPT)
> > 
> > - pid -> Is an alias for the correct config to enable PID tracing
> >   on any kernel.
> >   i.e, in EL1 kernel -> pid == contextid
> >           EL2 kernel -> pid == contextid_in_vmid
> > 
> > With this, the perf tool is also updated to request the "pid"
> > tracing whenever available, falling back to "contextid" if it
> > is unavailable.
> > 
> > Comparing against the old version, this patch series uses the metadata
> > to save PID format; after add new item into metadata, it introduces
> > backward compatibility issue.  To allow backward compatibility, this
> > series calculates per CPU metadata array size and avoid to use the
> > defined macro, so can always know the correct array size based on the
> > info stored in perf data file.  Finally, the PID format stored in
> > metadata is passed to decoder and guide the decoder to set PID from
> > CONTEXTIDR_EL1 or VMID.
> > 
> > This patch series has been tested on Arm Juno-r2 board, with testing
> > two perf data files: one data file is recorded by the latest perf tool
> > after applied this patch series, and another data file is recorded by
> > old perf tool without this patch series, so this can prove the tool is
> > backward compatible.
> > 
> > Changes from RFC:
> > * Added comments to clarify cases requested (Leo);
> > * Explain the change to generic flags for cs_etm_set_option() in the
> >   commit description;
> > * Stored PID format in metadata and passed it to decoder (Leo);
> > * Enhanced cs-etm for backward compatibility (Denis Nikitin).
> > 
> > [1] https://archive.armlinux.org.uk/lurker/message/20201110.183310.24406f33.en.html
> > 
> > 
> > Leo Yan (4):
> >   perf cs-etm: Calculate per CPU metadata array size
> >   perf cs-etm: Add PID format into metadata
> >   perf cs-etm: Fixup PID_FMT when it is zero
> >   perf cs-etm: Add helper cs_etm__get_pid_fmt()
> > 
> > Suzuki K Poulose (3):
> >   coresight: etm-perf: Add support for PID tracing for kernel at EL2
> >   perf cs_etm: Use pid tracing explicitly instead of contextid
> >   perf cs-etm: Detect pid in VMID for kernel running at EL2
> > 
> >  .../hwtracing/coresight/coresight-etm-perf.c  | 14 +++
> >  .../coresight/coresight-etm4x-core.c          |  9 ++
> >  include/linux/coresight-pmu.h                 | 11 ++-
> >  tools/include/linux/coresight-pmu.h           | 11 ++-
> >  tools/perf/arch/arm/util/cs-etm.c             | 89 +++++++++++++++----
> >  .../perf/util/cs-etm-decoder/cs-etm-decoder.c | 32 ++++++-
> >  tools/perf/util/cs-etm.c                      | 61 ++++++++++++-
> >  tools/perf/util/cs-etm.h                      |  3 +
> >  8 files changed, 198 insertions(+), 32 deletions(-)
> > 
> > -- 
> > 2.25.1
> >