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

Mathieu Poirier
2021-01-11T18:17:31Z

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.

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
>