KVM: Disable manual dirty log when dirty ring enabled

KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 is for KVM_CLEAR_DIRTY_LOG, which is only
useful for KVM_GET_DIRTY_LOG.  Skip enabling it for kvm dirty ring.

More importantly, KVM_DIRTY_LOG_INITIALLY_SET will not wr-protect all the pages
initially, which is against how kvm dirty ring is used - there's no way for kvm
dirty ring to re-protect a page before it's notified as being written first
with a GFN entry in the ring!  So when KVM_DIRTY_LOG_INITIALLY_SET is enabled
with dirty ring, we'll see silent data loss after migration.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210506160549.130416-10-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 23 insertions, 14 deletions

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 5afe15a..e1a6c7c 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2131,20 +2131,29 @@ static int kvm_init(MachineState *ms)
     s->coalesced_pio = s->coalesced_mmio &&
                        kvm_check_extension(s, KVM_CAP_COALESCED_PIO);
 
-    dirty_log_manual_caps =
-        kvm_check_extension(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2);
-    dirty_log_manual_caps &= (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE |
-                              KVM_DIRTY_LOG_INITIALLY_SET);
-    s->manual_dirty_log_protect = dirty_log_manual_caps;
-    if (dirty_log_manual_caps) {
-        ret = kvm_vm_enable_cap(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, 0,
-                                   dirty_log_manual_caps);
-        if (ret) {
-            warn_report("Trying to enable capability %"PRIu64" of "
-                        "KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 but failed. "
-                        "Falling back to the legacy mode. ",
-                        dirty_log_manual_caps);
-            s->manual_dirty_log_protect = 0;
+    /*
+     * KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 is not needed when dirty ring is
+     * enabled.  More importantly, KVM_DIRTY_LOG_INITIALLY_SET will assume no
+     * page is wr-protected initially, which is against how kvm dirty ring is
+     * usage - kvm dirty ring requires all pages are wr-protected at the very
+     * beginning.  Enabling this feature for dirty ring causes data corruption.
+     */
+    if (!s->kvm_dirty_ring_size) {
+        dirty_log_manual_caps =
+            kvm_check_extension(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2);
+        dirty_log_manual_caps &= (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE |
+                                  KVM_DIRTY_LOG_INITIALLY_SET);
+        s->manual_dirty_log_protect = dirty_log_manual_caps;
+        if (dirty_log_manual_caps) {
+            ret = kvm_vm_enable_cap(s, KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2, 0,
+                                    dirty_log_manual_caps);
+            if (ret) {
+                warn_report("Trying to enable capability %"PRIu64" of "
+                            "KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 but failed. "
+                            "Falling back to the legacy mode. ",
+                            dirty_log_manual_caps);
+                s->manual_dirty_log_protect = 0;
+            }
         }
     }