[PATCH] perf/x86/amd: Reject AMD Merge event as a countable event

[Luka Absandze <absandze@amazon.de>]

AMD event select 0xFFF (Merge) is the hardware control encoding used by
Large Increment per Cycle (LIPC) pairs.  When written into an odd
counter's PerfCtl it turns that counter into the upper 16 bits of a
64-bit merged accumulator; any subsequent arming write (-(2^47-1)) is
silently truncated to its low 16 bits, while hwc->prev_count retains the
full value.

This is particularly problematic with a KVM guest, which will program
the Merge encoding into its own odd vPMC as a normal part of scheduling
a LIPC event.  KVM intercepts that write and faithfully forwards it to
the host as a raw event carrying the Merge encoding, where it is
accepted as an ordinary countable event. Once a counter's PerfCtl holds
the Merge encoding, every later user of that counter gets a first delta
inflated by 2^47

Reject Merge (AMD_MERGE_EVENT) in amd_core_hw_config() so it cannot be
submitted as a user-visible event.  The pair mechanism programs it
itself via x86_pmu.perf_ctr_pair_en; nothing else should do so.

Fixes: 5738891229a2 ("perf/x86/amd: Add support for Large Increment per Cycle Events")
Reported-by: Yao Zhou <yaoz@meta.com>
Signed-off-by: Luka Absandze <absandze@amazon.de>
---
 arch/x86/events/amd/core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
index 49b6b8fce566..cb655c751e5b 100644
--- a/arch/x86/events/amd/core.c
+++ b/arch/x86/events/amd/core.c
@@ -380,8 +380,12 @@ static int amd_core_hw_config(struct perf_event *event)
 	else if (event->attr.exclude_guest)
 		event->hw.config |= AMD64_EVENTSEL_HOSTONLY;
 
-	if ((x86_pmu.flags & PMU_FL_PAIR) && amd_is_pair_event_code(&event->hw))
-		event->hw.flags |= PERF_X86_EVENT_PAIR;
+	if (x86_pmu.flags & PMU_FL_PAIR) {
+		if ((event->hw.config & AMD64_EVENTSEL_EVENT) == AMD_MERGE_EVENT)
+			return -EINVAL;
+		if (amd_is_pair_event_code(&event->hw))
+			event->hw.flags |= PERF_X86_EVENT_PAIR;
+	}
 
 	if (has_branch_stack(event))
 		return static_call(amd_pmu_branch_hw_config)(event);
-- 
2.47.3