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>

1 files changed, 13 insertions, 0 deletions

diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index 1b41e71283d5..c9e94d6503af 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/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
  *