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From: Peter Xu <peterx@redhat.com>
To: qemu-devel@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Keqian Zhu <zhukeqian1@huawei.com>,
	Hyman <huangy81@chinatelecom.cn>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	peterx@redhat.com
Subject: [PATCH v6 09/10] KVM: Disable manual dirty log when dirty ring enabled
Date: Wed, 24 Mar 2021 14:39:53 -0400	[thread overview]
Message-ID: <20210324183954.345629-10-peterx@redhat.com> (raw)
In-Reply-To: <20210324183954.345629-1-peterx@redhat.com>

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>
---
 accel/kvm/kvm-all.c | 37 +++++++++++++++++++++++--------------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 8baa002c009..daf59836ae2 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -2169,20 +2169,29 @@ static int kvm_init(MachineState *ms)
         }
     }
 
-    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_enabled) {
+        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;
+            }
         }
     }
 
-- 
2.26.2



  parent reply	other threads:[~2021-03-24 18:47 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-24 18:39 [PATCH v6 00/10] KVM: Dirty ring support (QEMU part) Peter Xu
2021-03-24 18:39 ` [PATCH v6 01/10] memory: Introduce log_sync_global() to memory listener Peter Xu
2021-03-24 18:39 ` [PATCH v6 02/10] KVM: Use a big lock to replace per-kml slots_lock Peter Xu
2021-03-24 18:39 ` [PATCH v6 03/10] KVM: Create the KVMSlot dirty bitmap on flag changes Peter Xu
2021-03-24 18:39 ` [PATCH v6 04/10] KVM: Provide helper to get kvm dirty log Peter Xu
2021-03-24 18:39 ` [PATCH v6 05/10] KVM: Provide helper to sync dirty bitmap from slot to ramblock Peter Xu
2021-03-24 18:39 ` [PATCH v6 06/10] KVM: Simplify dirty log sync in kvm_set_phys_mem Peter Xu
2021-03-24 18:39 ` [PATCH v6 07/10] KVM: Cache kvm slot dirty bitmap size Peter Xu
2021-03-24 18:39 ` [PATCH v6 08/10] KVM: Add dirty-gfn-count property Peter Xu
2021-03-24 18:39 ` Peter Xu [this message]
2021-03-24 18:41 ` [PATCH v6 10/10] KVM: Dirty ring support Peter Xu
2021-04-29 21:07 ` [PATCH v6 00/10] KVM: Dirty ring support (QEMU part) Peter Xu

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