[PATCH v3 5/5] KVM: MMU: fast drop all spte on the pte_list

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

If a shadow page is being zapped or a host page is going to be freed, kvm
will drop all the reverse-mappings on the shadow page or the gfn. Currently,
it drops the reverse-mapping one by one - it deletes the first reverse mapping,
then moves other reverse-mapping between the description-table. When the
last description-table become empty, it will be freed.

It works well if we only have a few reverse-mappings, but some pte_lists are
very long, during my tracking, i saw some gfns have more than 200 sptes listed
on its pte-list (1G memory in guest on softmmu). Optimization for dropping such
long pte-list is worthwhile, at lease it is good for deletion memslots and
ksm/thp merge pages.

This patch introduce a better way to optimize for this case, it walks all the
reverse-mappings and clear them, then free all description-tables together.

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
---
 arch/x86/kvm/mmu.c |   36 +++++++++++++++++++++++++++---------
 1 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 58f813a..aa7a887 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -945,6 +945,25 @@ static void pte_list_remove(u64 *spte, unsigned long *pte_list)
 	}
 }

+static void pte_list_destroy(unsigned long *pte_list)
+{
+	struct pte_list_desc *desc;
+	unsigned long list_value = *pte_list;
+
+	*pte_list = 0;
+
+	if (!(list_value & 1))
+		return;
+
+	desc = (struct pte_list_desc *)(list_value & ~1ul);
+	while (desc) {
+		struct pte_list_desc *next_desc = desc->more;
+
+		mmu_free_pte_list_desc(desc);
+		desc = next_desc;
+	}
+}
+
 /*
  * Used by the following functions to iterate through the sptes linked by a
  * pte_list.  All fields are private and not assumed to be used outside.
@@ -1183,17 +1202,17 @@ static bool rmap_write_protect(struct kvm *kvm, u64 gfn)
 static int kvm_unmap_rmapp(struct kvm *kvm, unsigned long *rmapp,
 			   struct kvm_memory_slot *slot, unsigned long data)
 {
-	u64 *sptep;
 	struct pte_list_iterator iter;
+	u64 *sptep;
 	int need_tlb_flush = 0;

-restart:
 	for_each_spte_in_rmap(*rmapp, iter, sptep) {
-		drop_spte(kvm, sptep);
+		mmu_spte_clear_track_bits(sptep);
 		need_tlb_flush = 1;
-		goto restart;
 	}

+	pte_list_destroy(rmapp);
+
 	return need_tlb_flush;
 }

@@ -2016,11 +2035,10 @@ static void kvm_mmu_unlink_parents(struct kvm *kvm, struct kvm_mmu_page *sp)
 	u64 *sptep;
 	struct pte_list_iterator iter;

-restart:
-	for_each_spte_in_pte_list(sp->parent_ptes, iter, sptep) {
-		drop_parent_pte(sp, sptep);
-		goto restart;
-	}
+	for_each_spte_in_pte_list(sp->parent_ptes, iter, sptep)
+		mmu_spte_clear_no_track(sptep);
+
+	pte_list_destroy(&sp->parent_ptes);
 }

 static int mmu_zap_unsync_children(struct kvm *kvm,
-- 
1.7.7.6