kvm: introduce manual dirty log reprotect

There are two problems with KVM_GET_DIRTY_LOG.  First, and less important,
it can take kvm->mmu_lock for an extended period of time.  Second, its user
can actually see many false positives in some cases.  The latter is due
to a benign race like this:

  1. KVM_GET_DIRTY_LOG returns a set of dirty pages and write protects
     them.
  2. The guest modifies the pages, causing them to be marked ditry.
  3. Userspace actually copies the pages.
  4. KVM_GET_DIRTY_LOG returns those pages as dirty again, even though
     they were not written to since (3).

This is especially a problem for large guests, where the time between
(1) and (3) can be substantial.  This patch introduces a new
capability which, when enabled, makes KVM_GET_DIRTY_LOG not
write-protect the pages it returns.  Instead, userspace has to
explicitly clear the dirty log bits just before using the content
of the page.  The new KVM_CLEAR_DIRTY_LOG ioctl can also operate on a
64-page granularity rather than requiring to sync a full memslot;
this way, the mmu_lock is taken for small amounts of time, and
only a small amount of time will pass between write protection
of pages and the sending of their content.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/Makefile

@@ -16,8 +16,10 @@ TEST_GEN_PROGS_x86_64 += x86_64/cr4_cpuid_sync_test
 TEST_GEN_PROGS_x86_64 += x86_64/state_test
 TEST_GEN_PROGS_x86_64 += x86_64/evmcs_test
 TEST_GEN_PROGS_x86_64 += dirty_log_test
+TEST_GEN_PROGS_x86_64 += clear_dirty_log_test
 
 TEST_GEN_PROGS_aarch64 += dirty_log_test
+TEST_GEN_PROGS_aarch64 += clear_dirty_log_test
 
 TEST_GEN_PROGS += $(TEST_GEN_PROGS_$(UNAME_M))
 LIBKVM += $(LIBKVM_$(UNAME_M))

tools/testing/selftests/kvm/clear_dirty_log_test.c

@@ -0,0 +1,2 @@
+#define USE_CLEAR_DIRTY_LOG
+#include "dirty_log_test.c"

tools/testing/selftests/kvm/dirty_log_test.c

@@ -275,6 +275,14 @@ static void run_test(enum vm_guest_mode mode, unsigned long iterations,
 
 	vm = create_vm(mode, VCPU_ID, guest_num_pages, guest_code);
 
+#ifdef USE_CLEAR_DIRTY_LOG
+	struct kvm_enable_cap cap = {};
+
+	cap.cap = KVM_CAP_MANUAL_DIRTY_LOG_PROTECT;
+	cap.args[0] = 1;
+	vm_enable_cap(vm, &cap);
+#endif
+
 	/* Add an extra memory slot for testing dirty logging */
 	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
 				    guest_test_mem,
@@ -316,6 +324,10 @@ static void run_test(enum vm_guest_mode mode, unsigned long iterations,
 		/* Give the vcpu thread some time to dirty some pages */
 		usleep(interval * 1000);
 		kvm_vm_get_dirty_log(vm, TEST_MEM_SLOT_INDEX, bmap);
+#ifdef USE_CLEAR_DIRTY_LOG
+		kvm_vm_clear_dirty_log(vm, TEST_MEM_SLOT_INDEX, bmap, 0,
+				       DIV_ROUND_UP(host_num_pages, 64) * 64);
+#endif
 		vm_dirty_log_verify(bmap);
 		iteration++;
 		sync_global_to_guest(vm, iteration);
@@ -392,6 +404,13 @@ int main(int argc, char *argv[])
 	unsigned int mode;
 	int opt, i;
 
+#ifdef USE_CLEAR_DIRTY_LOG
+	if (!kvm_check_cap(KVM_CAP_MANUAL_DIRTY_LOG_PROTECT)) {
+		fprintf(stderr, "KVM_CLEAR_DIRTY_LOG not available, skipping tests\n");
+		exit(KSFT_SKIP);
+	}
+#endif
+
 	while ((opt = getopt(argc, argv, "hi:I:o:tm:")) != -1) {
 		switch (opt) {
 		case 'i':

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

@@ -58,6 +58,8 @@ void kvm_vm_free(struct kvm_vm *vmp);
 void kvm_vm_restart(struct kvm_vm *vmp, int perm);
 void kvm_vm_release(struct kvm_vm *vmp);
 void kvm_vm_get_dirty_log(struct kvm_vm *vm, int slot, void *log);
+void kvm_vm_clear_dirty_log(struct kvm_vm *vm, int slot, void *log,
+			    uint64_t first_page, uint32_t num_pages);
 
 int kvm_memcmp_hva_gva(void *hva, struct kvm_vm *vm, const vm_vaddr_t gva,
 		       size_t len);

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

@@ -231,6 +231,19 @@ void kvm_vm_get_dirty_log(struct kvm_vm *vm, int slot, void *log)
 		    strerror(-ret));
 }
 
+void kvm_vm_clear_dirty_log(struct kvm_vm *vm, int slot, void *log,
+			    uint64_t first_page, uint32_t num_pages)
+{
+	struct kvm_clear_dirty_log args = { .dirty_bitmap = log, .slot = slot,
+		                            .first_page = first_page,
+	                                    .num_pages = num_pages };
+	int ret;
+
+	ret = ioctl(vm->fd, KVM_CLEAR_DIRTY_LOG, &args);
+	TEST_ASSERT(ret == 0, "%s: KVM_CLEAR_DIRTY_LOG failed: %s",
+		    strerror(-ret));
+}
+
 /*
  * Userspace Memory Region Find
  *