KVM: selftests: Ensure guest writes min number of pages in dirty_log_test

Ensure the vCPU fully completes at least one write in each dirty_log_test
iteration, as failure to dirty any pages complicates verification and
forces the test to be overly conservative about possible values.  E.g.
verification needs to allow the last dirty page from a previous iteration
to have *any* value, because the vCPU could get stuck for multiple
iterations, which is unlikely but can happen in heavily overloaded and/or
nested virtualization setups.

Somewhat arbitrarily set the minimum to 0x100/256; high enough to be
interesting, but not so high as to lead to pointlessly long runtimes.

Opportunistically report the number of writes per iteration for debug
purposes, and so that a human can sanity check the test.  Due to each
write targeting a random page, the number of dirty pages will likely be
lower than the number of total writes, but it shouldn't be absurdly lower
(which would suggest the pRNG is broken)

Reported-by: Maxim Levitsky <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

tools/testing/selftests/kvm/dirty_log_test.c

@@ -37,6 +37,12 @@
 /* Interval for each host loop (ms) */
 #define TEST_HOST_LOOP_INTERVAL		10UL
 
+/*
+ * Ensure the vCPU is able to perform a reasonable number of writes in each
+ * iteration to provide a lower bound on coverage.
+ */
+#define TEST_MIN_WRITES_PER_ITERATION	0x100
+
 /* Dirty bitmaps are always little endian, so we need to swap on big endian */
 #if defined(__s390x__)
 # define BITOP_LE_SWIZZLE	((BITS_PER_LONG-1) & ~0x7)
@@ -72,6 +78,7 @@ static uint64_t host_page_size;
 static uint64_t guest_page_size;
 static uint64_t guest_num_pages;
 static uint64_t iteration;
+static uint64_t nr_writes;
 static bool vcpu_stop;
 
 /*
@@ -107,6 +114,7 @@ static void guest_code(void)
 	for (i = 0; i < guest_num_pages; i++) {
 		addr = guest_test_virt_mem + i * guest_page_size;
 		vcpu_arch_put_guest(*(uint64_t *)addr, READ_ONCE(iteration));
+		nr_writes++;
 	}
 #endif
 
@@ -118,6 +126,7 @@ static void guest_code(void)
 			addr = align_down(addr, host_page_size);
 
 			vcpu_arch_put_guest(*(uint64_t *)addr, READ_ONCE(iteration));
+			nr_writes++;
 		}
 
 		GUEST_SYNC(1);
@@ -548,8 +557,8 @@ static void vm_dirty_log_verify(enum vm_guest_mode mode, unsigned long **bmap)
 		}
 	}
 
-	pr_info("Iteration %2ld: dirty: %-6lu clean: %-6lu\n",
-		iteration, nr_dirty_pages, nr_clean_pages);
+	pr_info("Iteration %2ld: dirty: %-6lu clean: %-6lu writes: %-6lu\n",
+		iteration, nr_dirty_pages, nr_clean_pages, nr_writes);
 
 	host_dirty_count += nr_dirty_pages;
 	host_clear_count += nr_clean_pages;
@@ -665,6 +674,8 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	host_dirty_count = 0;
 	host_clear_count = 0;
 	WRITE_ONCE(dirty_ring_vcpu_ring_full, false);
+	WRITE_ONCE(nr_writes, 0);
+	sync_global_to_guest(vm, nr_writes);
 
 	/*
 	 * Ensure the previous iteration didn't leave a dangling semaphore, i.e.
@@ -683,10 +694,22 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 
 		dirty_ring_prev_iteration_last_page = dirty_ring_last_page;
 
-		/* Give the vcpu thread some time to dirty some pages */
-		for (i = 0; i < p->interval; i++) {
+		/*
+		 * Let the vCPU run beyond the configured interval until it has
+		 * performed the minimum number of writes.  This verifies the
+		 * guest is making forward progress, e.g. isn't stuck because
+		 * of a KVM bug, and puts a firm floor on test coverage.
+		 */
+		for (i = 0; i < p->interval || nr_writes < TEST_MIN_WRITES_PER_ITERATION; i++) {
+			/*
+			 * Sleep in 1ms chunks to keep the interval math simple
+			 * and so that the test doesn't run too far beyond the
+			 * specified interval.
+			 */
 			usleep(1000);
 
+			sync_global_from_guest(vm, nr_writes);
+
 			/*
 			 * Reap dirty pages while the guest is running so that
 			 * dirty ring full events are resolved, i.e. so that a
@@ -737,6 +760,12 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 		WRITE_ONCE(vcpu_stop, false);
 		sync_global_to_guest(vm, vcpu_stop);
 
+		/*
+		 * Sync the number of writes performed before verification, the
+		 * info will be printed along with the dirty/clean page counts.
+		 */
+		sync_global_from_guest(vm, nr_writes);
+
 		/*
 		 * NOTE: for dirty ring, it's possible that we didn't stop at
 		 * GUEST_SYNC but instead we stopped because ring is full;
@@ -760,6 +789,9 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 			WRITE_ONCE(host_quit, true);
 		sync_global_to_guest(vm, iteration);
 
+		WRITE_ONCE(nr_writes, 0);
+		sync_global_to_guest(vm, nr_writes);
+
 		WRITE_ONCE(dirty_ring_vcpu_ring_full, false);
 
 		sem_post(&sem_vcpu_cont);