KVM: selftests: Print the seed for the guest pRNG iff it has changed

Print the guest's random seed during VM creation if and only if the seed
has changed since the seed was last printed.  The vast majority of tests,
if not all tests at this point, set the seed during test initialization
and never change the seed, i.e. printing it every time a VM is created is
useless noise.

Snapshot and print the seed during early selftest init to play nice with
tests that use the kselftests harness, at the cost of printing an unused
seed for tests that change the seed during test-specific initialization,
e.g. dirty_log_perf_test.  The kselftests harness runs each testcase in a
separate process that is forked from the original process before creating
each testcase's VM, i.e. waiting until first VM creation will result in
the seed being printed by each testcase despite it never changing.  And
long term, the hope/goal is that setting the seed will be handled by the
core framework, i.e. that the dirty_log_perf_test wart will naturally go
away.

Reported-by: Yi Lai <yi1.lai@intel.com>
Reported-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240627021756.144815-2-dapeng1.mi@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index ad00e4761886..56b170b725b3 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -21,6 +21,7 @@
 
 uint32_t guest_random_seed;
 struct guest_random_state guest_rng;
+static uint32_t last_guest_seed;
 
 static int vcpu_mmap_sz(void);
 
@@ -434,7 +435,10 @@ struct kvm_vm *__vm_create(struct vm_shape shape, uint32_t nr_runnable_vcpus,
 	slot0 = memslot2region(vm, 0);
 	ucall_init(vm, slot0->region.guest_phys_addr + slot0->region.memory_size);
 
-	pr_info("Random seed: 0x%x\n", guest_random_seed);
+	if (guest_random_seed != last_guest_seed) {
+		pr_info("Random seed: 0x%x\n", guest_random_seed);
+		last_guest_seed = guest_random_seed;
+	}
 	guest_rng = new_guest_random_state(guest_random_seed);
 	sync_global_to_guest(vm, guest_rng);
 
@@ -2319,7 +2323,8 @@ void __attribute((constructor)) kvm_selftest_init(void)
 	/* Tell stdout not to buffer its content. */
 	setbuf(stdout, NULL);
 
-	guest_random_seed = random();
+	guest_random_seed = last_guest_seed = random();
+	pr_info("Random seed: 0x%x\n", guest_random_seed);
 
 	kvm_selftest_arch_init();
 }