[PATCH 10/20] KVM: selftests: Keep dirty_log_test vCPU in guest until it needs to stop

Sean Christopherson
2024-12-14T01:07:42Z

In the dirty_log_test guest code, exit to userspace only when the vCPU is
explicitly told to stop. Periodically exiting just to check if a flag has
been set is unnecessary, weirdly complex, and wastes time handling exits
that could be used to dirty memory.

Opportunistically convert 'i' to a uint64_t to guard against the unlikely
scenario that guest_num_pages exceeds the storage of an int.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
tools/testing/selftests/kvm/dirty_log_test.c | 43 ++++++++++----------
1 file changed, 22 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index 8d31e275a23d..40c8f5551c8e 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -31,9 +31,6 @@
/* Default guest test virtual memory offset */
#define DEFAULT_GUEST_TEST_MEM 0xc0000000

-/* How many pages to dirty for each guest loop */
-#define TEST_PAGES_PER_LOOP 1024
-
/* How many host loops to run (one KVM_GET_DIRTY_LOG for each loop) */
#define TEST_HOST_LOOP_N 32UL

@@ -75,6 +72,7 @@ static uint64_t host_page_size;
static uint64_t guest_page_size;
static uint64_t guest_num_pages;
static uint64_t iteration;
+static bool vcpu_stop;

/*
* Guest physical memory offset of the testing memory slot.
@@ -96,9 +94,10 @@ static uint64_t guest_test_virt_mem = DEFAULT_GUEST_TEST_MEM;
static void guest_code(void)
{
uint64_t addr;
- int i;

#ifdef __s390x__
+ uint64_t i;
+
/*
* On s390x, all pages of a 1M segment are initially marked as dirty
* when a page of the segment is written to for the very first time.
@@ -112,7 +111,7 @@ static void guest_code(void)
#endif

while (true) {
- for (i = 0; i < TEST_PAGES_PER_LOOP; i++) {
+ while (!READ_ONCE(vcpu_stop)) {
addr = guest_test_virt_mem;
addr += (guest_random_u64(&guest_rng) % guest_num_pages)
* guest_page_size;
@@ -140,14 +139,7 @@ static uint64_t host_track_next_count;
/* Whether dirty ring reset is requested, or finished */
static sem_t sem_vcpu_stop;
static sem_t sem_vcpu_cont;
-/*
- * This is only set by main thread, and only cleared by vcpu thread. It is
- * used to request vcpu thread to stop at the next GUEST_SYNC, since GUEST_SYNC
- * is the only place that we'll guarantee both "dirty bit" and "dirty data"
- * will match. E.g., SIG_IPI won't guarantee that if the vcpu is interrupted
- * after setting dirty bit but before the data is written.
- */
-static atomic_t vcpu_sync_stop_requested;
+
/*
* This is updated by the vcpu thread to tell the host whether it's a
* ring-full event. It should only be read until a sem_wait() of
@@ -272,9 +264,7 @@ static void clear_log_collect_dirty_pages(struct kvm_vcpu *vcpu, int slot,
/* Should only be called after a GUEST_SYNC */
static void vcpu_handle_sync_stop(void)
{
- if (atomic_read(&vcpu_sync_stop_requested)) {
- /* It means main thread is sleeping waiting */
- atomic_set(&vcpu_sync_stop_requested, false);
+ if (READ_ONCE(vcpu_stop)) {
sem_post(&sem_vcpu_stop);
sem_wait(&sem_vcpu_cont);
}
@@ -801,11 +791,24 @@ static void run_test(enum vm_guest_mode mode, void *arg)
}

/*
- * See vcpu_sync_stop_requested definition for details on why
- * we need to stop vcpu when verify data.
+ * Stop the vCPU prior to collecting and verifying the dirty
+ * log. If the vCPU is allowed to run during collection, then
+ * pages that are written during this iteration may be missed,
+ * i.e. collected in the next iteration. And if the vCPU is
+ * writing memory during verification, pages that this thread
+ * sees as clean may be written with this iteration's value.
*/
- atomic_set(&vcpu_sync_stop_requested, true);
+ WRITE_ONCE(vcpu_stop, true);
+ sync_global_to_guest(vm, vcpu_stop);
sem_wait(&sem_vcpu_stop);
+
+ /*
+ * Clear vcpu_stop after the vCPU thread has acknowledge the
+ * stop request and is waiting, i.e. is definitely not running!
+ */
+ WRITE_ONCE(vcpu_stop, false);
+ sync_global_to_guest(vm, vcpu_stop);
+
/*
* NOTE: for dirty ring, it's possible that we didn't stop at
* GUEST_SYNC but instead we stopped because ring is full;
@@ -813,8 +816,6 @@ static void run_test(enum vm_guest_mode mode, void *arg)
* the flush of the last page, and since we handle the last
* page specially verification will succeed anyway.
*/
- assert(host_log_mode == LOG_MODE_DIRTY_RING ||
- atomic_read(&vcpu_sync_stop_requested) == false);
vm_dirty_log_verify(mode, bmap);

/*
--
2.47.1.613.gc27f4b7a9f-goog