On 07/30/2013 09:27 PM, Paolo Bonzini wrote:
> Il 30/07/2013 15:02, Xiao Guangrong ha scritto:
>> kvm_vm_ioctl_get_dirty_log() write-protects the spte based on the dirty
>> bitmap, we should ensure the writable spte can be found in rmap before the
>> dirty bitmap is visible. Otherwise, we cleared the dirty bitmap and failed
>> to write-protect the page
>>
>> It need the memory barrier to prevent out-of-order that will be added in the
>> later patch
>
> Do you mean that the later patch will also introduce a memory barrier?
No. Sorry for the confusion. I mean we miss the memory barrier in this patch
and will fix it in the latter patch where we introduce the lockless
write-protection.
The memory barrier is added in
[PATCH 11/12] KVM: MMU: locklessly write-protect the page:
+ /*
+ * We should put the sptep into rmap before dirty log
+ * otherwise the lockless spte write-protect path will
+ * clear the dirty bit map but fail to find the spte.
+ *
+ * See the comments in kvm_vm_ioctl_get_dirty_log().
+ */
+ smp_wmb();
+
if (pte_access & ACC_WRITE_MASK)
and the barrier in the another side is:
+ /*
+ * xchg acts as a full barrier that ensures
+ * clearing dirty bitmap before read rmap.
+ *
+ * See the comments in set_spte().
+ */
mask = xchg(&dirty_bitmap[i], 0);
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
