KVM: selftests: Provide an API for getting a random bool from an RNG

Move memstress' random bool logic into common code to avoid reinventing
the wheel for basic yes/no decisions.  Provide an outer wrapper to handle
the basic/common case of just wanting a 50/50 chance of something
happening.

Link: https://lore.kernel.org/r/20240314185459.2439072-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/test_util.h

@@ -97,6 +97,17 @@ extern struct guest_random_state guest_rng;
 struct guest_random_state new_guest_random_state(uint32_t seed);
 uint32_t guest_random_u32(struct guest_random_state *state);
 
+static inline bool __guest_random_bool(struct guest_random_state *state,
+				       uint8_t percent)
+{
+	return (guest_random_u32(state) % 100) < percent;
+}
+
+static inline bool guest_random_bool(struct guest_random_state *state)
+{
+	return __guest_random_bool(state, 50);
+}
+
 static inline uint64_t guest_random_u64(struct guest_random_state *state)
 {
 	return ((uint64_t)guest_random_u32(state) << 32) | guest_random_u32(state);

tools/testing/selftests/kvm/lib/memstress.c

@@ -74,7 +74,7 @@ void memstress_guest_code(uint32_t vcpu_idx)
 
 			addr = gva + (page * args->guest_page_size);
 
-			if (guest_random_u32(&rand_state) % 100 < args->write_percent)
+			if (__guest_random_bool(&rand_state, args->write_percent))
 				*(uint64_t *)addr = 0x0123456789ABCDEF;
 			else
 				READ_ONCE(*(uint64_t *)addr);