Re: [PATCH v1 1/7] coresight: etm-perf: Add support for PID tracing for kernel at EL2

Hi Leo,

On Tue, 12 Jan 2021 at 08:58, Leo Yan [off-list ref] wrote:

Hi Mike,

On Mon, Jan 11, 2021 at 04:22:39PM +0000, Mike Leach wrote:

[...]

quoted

quoted

diff --git a/include/linux/coresight-pmu.h b/include/linux/coresight-pmu.h
index b0e35eec6499..927c6285ce5d 100644
--- a/include/linux/coresight-pmu.h
+++ b/include/linux/coresight-pmu.h

@@ -11,16 +11,19 @@
 #define CORESIGHT_ETM_PMU_SEED  0x10

 /* ETMv3.5/PTM's ETMCR config bit */
-#define ETM_OPT_CYCACC  12
-#define ETM_OPT_CTXTID 14
-#define ETM_OPT_TS      28
-#define ETM_OPT_RETSTK 29
+#define ETM_OPT_CYCACC         12
+#define ETM_OPT_CTXTID         14
+#define ETM_OPT_CTXTID_IN_VMID 15

Minor issue here - ETMv3.x / PTM cannot trace CXTID in VMID so this
may better be named ETM4_OPT_CTXTID_IN_VMID, rather than be grouped
with the ETM3.5 options?

I looked into this suggestion but found it's complex than I assumed.
This config bits are not only used for ETMv3.x / PTM, it's also used
as an configuration interface between user space in Perf and kernel
drivers.

For example, in the userspace, perf tool sets bit ETM_OPT_TS to enable
timestamp [1], this is same for ETMv3 and ETMv4.  In the kernel side,
the configuration is directly used ETMv3 (in coresight-etm3x-core.c),
but the configuration bits are converted for ETMv4 in the function
etm4_parse_event_config() [2].

So this is a historical issue, at the early period ETMv3 and ETMv4 can
be compatible with each other for configurations, but after evoluation,
some configs only belong to ETMv4 and cannot be applied on ETMv3
anymore, but we still use ETMv3.5 config bits as the interface between
kernel and userspace.

I was aware that etm3/ptm used these bits as both the options and the
bit values for direct writing to the ETMCR register for ETMv3, and
re-mapped to appropriate register values in ETMv4.
In the past we have re-used etmv3.5 bit definitions ETM_xxx  when
appropriate, but where unique to ETM4 we  have used a ETM4_xxx naming
convention.

I am not suggesting re-factoring the options completely, just
re-naming this single option to make it clear it is unique to ETM4+.

Looking at the etmv3 driver, at present it does not actually appear to
support contextid tracing - and when it does, both bits 14 and 15 will
be required to be used - as ETMCR defines these bits as ContextID
size.
Should this ever get fixed. then having an overlapping option bit -
that appears to be valid for ETMv3 will be confusing.

Regards

Mike

I'd like suggest we use a saperate patch set to refactor the
configuration bits, e.g. define the arbitrary configuration bits as
interface between user space and kernel, these bits are neutral for any
ETM version, in the kernel ETM drivers need to convert to its own
configuration formats.  For this patch, I want to keep current
change as it is, is this okay for you?

Thanks,
Leo

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm/util/cs-etm.c?h=v5.11-rc3#n152
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/hwtracing/coresight/coresight-etm4x-core.c?h=v5.11-rc3#n422

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

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