[PATCHv2 2/7] KVM: PPC: Book3S: Don't call __ffs() on an empty pending_exceptions bitmap

[Ritesh Harjani (IBM)]

This problem was caught when running KVM PPC selftests on big-endian.
__ffs() doc clearly says:
  "Undefined if no bit exists, so code should check against 0 first"

This cause KVM to deliever an arbitrary interrupt to the guest and was
causing guest to hang up while running these selftests.

On checking further - GCC was emitting 0 in this case, which caused
System Reset in guest while running kvm selftests.

Use find_first_bit() instead, which reads the word once and returns the
bitmap size when no bit is set, instead of reading *pending twice
thereby causing TOCTOU as pointed by Sashiko.

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
---
 arch/powerpc/kvm/book3s.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
index 2efbe05caed7..38816578d002 100644
--- a/arch/powerpc/kvm/book3s.c
+++ b/arch/powerpc/kvm/book3s.c
@@ -401,7 +401,7 @@ int kvmppc_core_prepare_to_enter(struct kvm_vcpu *vcpu)
 	if (vcpu->arch.pending_exceptions)
 		printk(KERN_EMERG "KVM: Check pending: %lx\n", vcpu->arch.pending_exceptions);
 #endif
-	priority = __ffs(*pending);
+	priority = find_first_bit(pending, BITS_PER_LONG * sizeof(*pending));
 	while (priority < BOOK3S_IRQPRIO_MAX) {
 		if (kvmppc_book3s_irqprio_deliver(vcpu, priority) &&
 		    clear_irqprio(vcpu, priority)) {
--
2.39.5