powerpc/vpa_dtl: Add interface to expose vpa dtl counters via perf

The pseries Shared Processor Logical Partition(SPLPAR) machines
can retrieve a log of dispatch and preempt events from the
hypervisor using data from Disptach Trace Log(DTL) buffer.
With this information, user can retrieve when and why each dispatch &
preempt has occurred. Added an interface to expose the Virtual Processor
Area(VPA) DTL counters via perf.

The following events are available and exposed in sysfs:

 vpa_dtl/dtl_cede/ - Trace voluntary (OS initiated) virtual processor waits
 vpa_dtl/dtl_preempt/ - Trace time slice preempts
 vpa_dtl/dtl_fault/ - Trace virtual partition memory page faults.
 vpa_dtl/dtl_all/ - Trace all (dtl_cede/dtl_preempt/dtl_fault)

Added interface defines supported event list, config fields for the
event attributes and their corresponding bit values which are exported
via sysfs. User could use the standard perf tool to access perf events
exposed via vpa-dtl pmu.

The VPA DTL PMU counters do not interrupt on overflow or generate any
PMI interrupts. Therefore, the kernel needs to poll the counters, added
hrtimer code to do that. The timer interval can be provided by user via
sample_period field in nano seconds. There is one hrtimer added per
vpa-dtl pmu thread.

To ensure there are no other conflicting dtl users (example: debugfs dtl
or /proc/powerpc/vcpudispatch_stats), interface added code to use
"down_write_trylock" call to take the dtl_access_lock. The dtl_access_lock
is defined in dtl.h file. Also added global reference count variable called
"dtl_global_refc", to ensure dtl data can be captured per-cpu. Code also
added global lock called "dtl_global_lock" to avoid race condition.

Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Tested-by: Tejas Manhas <tejas05@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250915102947.26681-3-atrajeev@linux.ibm.com

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