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Subject[PATCH 0/3] perf_events: update extra shared registers management (v3)

The following short series of patches improves the code
which manages the extra shared regs used by some events
on Intel processors. Those events require an extra MSR
which may be shared between siblings CPUs when HT is on.
When HT is off, the kernel still needs to ensure that
events within an event group do not try to program
different values into that extra MSR.

This series improves the current code for managing the
register sharing by using static allocation instead of
dynamically trying to find a table slot to host that
extra MSR. This greatly simplifies the code. The patch
also prepare the kernel for more registers with those
kinds of constraints (e.g, LBR_SELECT, LD_LAT).

The patch also adds the missing group validation of
events using those extra MSRs. Up until now, one could
put two instances of the those events which had incompatible
values for the extra MSR. There was no upfront check and
the group would never be scheduled. Now, such group cannot
be constructed anymore (fail early).

Finally, the third patch adds the SandyBridge support for
the offcore_response events (which use these shared MSR).
It also removes the offcore_response events from the
SandyBridge constraint event table. Those events don't
have any constraints contrary to what's published in
the documentation.

The second version updates PATCH 1/3 which was an
older version with reg->idx initialization problems.

The third version solves the lockdep issues by using
spin_lock_irqsave in __intel_shared_reg_get_constraints().

[PATCH 0/3] introduction
[PATCH 1/3] rework of the register sharing logic
[PATCH 2/3] add missing shared regs validation
[PATCH 3/3] add Intel SandyBridge offcore_response support

Signed-off-by: Stephane Eranian <eranian@google.com>
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