kvm: x86/pmu: Fix the compare function used by the pmu event filter

When returning from the compare function the u64 is truncated to an
int.  This results in a loss of the high nybble[1] in the event select
and its sign if that nybble is in use.  Switch from using a result that
can end up being truncated to a result that can only be: 1, 0, -1.

[1] bits 35:32 in the event select register and bits 11:8 in the event
    select.

Fixes: 7ff775a ("KVM: x86/pmu: Use binary search to check filtered events")
Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220517051238.2566934-1-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: suomilewis

arch/x86/kvm/pmu.c

@@ -171,9 +171,12 @@ static bool pmc_resume_counter(struct kvm_pmc *pmc)
 	return true;
 }
 
-static int cmp_u64(const void *a, const void *b)
+static int cmp_u64(const void *pa, const void *pb)
 {
-	return *(__u64 *)a - *(__u64 *)b;
+	u64 a = *(u64 *)pa;
+	u64 b = *(u64 *)pb;
+
+	return (a > b) - (a < b);
 }
 
 void reprogram_gp_counter(struct kvm_pmc *pmc, u64 eventsel)