kvm: mmu: lazy collapse small sptes into large sptes
Dirty logging tracks sptes in 4k granularity, meaning that large sptes
have to be split. If live migration is successful, the guest in the
source machine will be destroyed and large sptes will be created in the
destination. However, the guest continues to run in the source machine
(for example if live migration fails), small sptes will remain around
and cause bad performance.
This patch introduce lazy collapsing of small sptes into large sptes.
The rmap will be scanned in ioctl context when dirty logging is stopped,
dropping those sptes which can be collapsed into a single large-page spte.
Later page faults will create the large-page sptes.
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
Message-Id: <1428046825-6905-1-git-send-email-wanpeng.li@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index faf044d..b8cb1d0 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7665,6 +7665,23 @@
new = id_to_memslot(kvm->memslots, mem->slot);
/*
+ * Dirty logging tracks sptes in 4k granularity, meaning that large
+ * sptes have to be split. If live migration is successful, the guest
+ * in the source machine will be destroyed and large sptes will be
+ * created in the destination. However, if the guest continues to run
+ * in the source machine (for example if live migration fails), small
+ * sptes will remain around and cause bad performance.
+ *
+ * Scan sptes if dirty logging has been stopped, dropping those
+ * which can be collapsed into a single large-page spte. Later
+ * page faults will create the large-page sptes.
+ */
+ if ((change != KVM_MR_DELETE) &&
+ (old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
+ !(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
+ kvm_mmu_zap_collapsible_sptes(kvm, new);
+
+ /*
* Set up write protection and/or dirty logging for the new slot.
*
* For KVM_MR_DELETE and KVM_MR_MOVE, the shadow pages of old slot have