KVM: arm64: PMU: Align chained counter implementation with architecture pseudocode

Ricardo recently pointed out that the PMU chained counter emulation
in KVM wasn't quite behaving like the one on actual hardware, in
the sense that a chained counter would expose an overflow on
both halves of a chained counter, while KVM would only expose the
overflow on the top half.

The difference is subtle, but significant. What does the architecture
say (DDI0087 H.a):

- Up to PMUv3p4, all counters but the cycle counter are 32bit

- A 32bit counter that overflows generates a CHAIN event on the
  adjacent counter after exposing its own overflow status

- The CHAIN event is accounted if the counter is correctly
  configured (CHAIN event selected and counter enabled)

This all means that our current implementation (which uses 64bit
perf events) prevents us from emulating this overflow on the lower half.

How to fix this? By implementing the above, to the letter.

This largely results in code deletion, removing the notions of
"counter pair", "chained counters", and "canonical counter".
The code is further restructured to make the CHAIN handling similar
to SWINC, as the two are now extremely similar in behaviour.

Reported-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Link: https://lore.kernel.org/r/20221113163832.3154370-3-maz@kernel.org

1 files changed, 0 insertions, 2 deletions

diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
index c0b868ce6a8f..96b192139a23 100644
--- a/include/kvm/arm_pmu.h
+++ b/include/kvm/arm_pmu.h
@@ -11,7 +11,6 @@
 #include <asm/perf_event.h>
 
 #define ARMV8_PMU_CYCLE_IDX		(ARMV8_PMU_MAX_COUNTERS - 1)
-#define ARMV8_PMU_MAX_COUNTER_PAIRS	((ARMV8_PMU_MAX_COUNTERS + 1) >> 1)
 
 #ifdef CONFIG_HW_PERF_EVENTS
 
@@ -29,7 +28,6 @@ struct kvm_pmu {
 	struct irq_work overflow_work;
 	struct kvm_pmu_events events;
 	struct kvm_pmc pmc[ARMV8_PMU_MAX_COUNTERS];
-	DECLARE_BITMAP(chained, ARMV8_PMU_MAX_COUNTER_PAIRS);
 	int irq_num;
 	bool created;
 	bool irq_level;