Re: [PATCH v2] kvm: mmu: lazy collapse small sptes into large sptes

From: Xiao Guangrong
Date: Fri Apr 03 2015 - 03:30:21 EST

On 04/03/2015 02:09 PM, Wanpeng Li wrote:
There are two scenarios for the requirement of collapsing small sptes
into large sptes.
- dirty logging tracks sptes in 4k granularity, so large sptes are split,
the large sptes will be reallocated in the destination machine and the
guest in the source machine will be destroyed when live migration successfully.
However, the guest in the source machine will continue to run if live migration
fail due to some reasons, the sptes still keep small which lead to bad
- our customers write tools to track the dirty speed of guests by EPT D bit/PML
in order to determine the most appropriate one to be live migrated, however
sptes will still keep small after tracking dirty speed.

This patch introduce lazy collapse small sptes into large sptes, the memory region
will be scanned on the ioctl context when dirty log is stopped, the ones which can
be collapsed into large pages will be dropped during the scan, it depends the on
later #PF to reallocate all large sptes.

Signed-off-by: Wanpeng Li <>
v1 -> v2:
* use 'bool' instead of 'int'
* add more comments
* fix can not get the next spte after drop the current spte

arch/x86/include/asm/kvm_host.h | 2 ++
arch/x86/kvm/mmu.c | 71 +++++++++++++++++++++++++++++++++++++++++
arch/x86/kvm/x86.c | 19 +++++++++++
3 files changed, 92 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 30b28dc..91b5bdb 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -854,6 +854,8 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
struct kvm_memory_slot *memslot);
+void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
+ struct kvm_memory_slot *memslot);
void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
struct kvm_memory_slot *memslot);
void kvm_mmu_slot_largepage_remove_write_access(struct kvm *kvm,
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index cee7592..df3f2e3 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4465,6 +4465,77 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm,

+static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
+ unsigned long *rmapp)
+ u64 *sptep;
+ struct rmap_iterator iter;
+ int need_tlb_flush = 0;
+ pfn_t pfn;
+ struct kvm_mmu_page *sp;
+ while ((sptep = rmap_get_first(*rmapp, &iter))) {
+ BUG_ON(!(*sptep & PT_PRESENT_MASK));
+ sp = page_header(__pa(sptep));
+ pfn = spte_to_pfn(*sptep);
+ /*
+ * Let support EPT only now, an efficient way need to be figure
+ * out to let these code be aware what mapping level used in
+ * guest.

This English seems strange... but i am not good at it. :)

+ */
+ if (sp-> &&
+ !kvm_is_reserved_pfn(pfn) &&
+ PageTransCompound(pfn_to_page(pfn))) {
+ drop_spte(kvm, sptep);
+ need_tlb_flush = 1;
+ }

If the conditions are not comfortable, it does loop forever...

Otherwise, it looks good to me.

Reviewed-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at