KVM: arm64: PMU: Fix period computation for 64bit counters with 32bit overflow

Fix the bogus masking when computing the period of a 64bit counter
with 32bit overflow. It really should be treated like a 32bit counter
for the purpose of the period.
Reported-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/Y4jbosgHbUDI0WF4@google.com

arch/arm64/kvm/pmu-emul.c

@@ -461,14 +461,10 @@ static u64 compute_period(struct kvm_pmc *pmc, u64 counter)
 {
 	u64 val;
 
-	if (kvm_pmc_is_64bit(pmc)) {
-		if (!kvm_pmc_has_64bit_overflow(pmc))
-			val = -(counter & GENMASK(31, 0));
-		else
-			val = (-counter) & GENMASK(63, 0);
-	} else {
+	if (kvm_pmc_is_64bit(pmc) && kvm_pmc_has_64bit_overflow(pmc))
+		val = (-counter) & GENMASK(63, 0);
+	else
 		val = (-counter) & GENMASK(31, 0);
-	}
 
 	return val;
 }