KVM: x86: Optimize mmio spte zapping when creating/moving memslot

When we create or move a memory slot, we need to zap mmio sptes.
Currently, zap_all() is used for this and this is causing two problems:
 - extra page faults after zapping mmu pages
 - long mmu_lock hold time during zapping mmu pages

For the latter, Marcelo reported a disastrous mmu_lock hold time during
hot-plug, which made the guest unresponsive for a long time.

This patch takes a simple way to fix these problems: do not zap mmu
pages unless they are marked mmio cached.  On our test box, this took
only 50us for the 4GB guest and we did not see ms of mmu_lock hold time
any more.

Note that we still need to do zap_all() for other cases.  So another
work is also needed: Xiao's work may be the one.

Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 9b75cae83d1014b6af45cea0ff55db7e67343fb3..3f205c6cde593f4f3bb2579146a79f14275d93d3 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -767,6 +767,7 @@ void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
                                     struct kvm_memory_slot *slot,
                                     gfn_t gfn_offset, unsigned long mask);
 void kvm_mmu_zap_all(struct kvm *kvm);
+void kvm_mmu_zap_mmio_sptes(struct kvm *kvm);
 unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages);
 

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index de45ec19534621c36d07243657a8955877fca518..c1a9b7b08ab75b39d649d3a1226df96637f8028e 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4189,6 +4189,24 @@ restart:
        spin_unlock(&kvm->mmu_lock);
 }
 
+void kvm_mmu_zap_mmio_sptes(struct kvm *kvm)
+{
+       struct kvm_mmu_page *sp, *node;
+       LIST_HEAD(invalid_list);
+
+       spin_lock(&kvm->mmu_lock);
+restart:
+       list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
+               if (!sp->mmio_cached)
+                       continue;
+               if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
+                       goto restart;
+       }
+
+       kvm_mmu_commit_zap_page(kvm, &invalid_list);
+       spin_unlock(&kvm->mmu_lock);
+}
+
 static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)
 {
        struct kvm *kvm;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 61a5bb60af86d58bfe1a40aa72739f532d7e2380..d3c478742e2ce5d6e45acb3b544c67171c4476a4 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6991,7 +6991,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
         * mmio sptes.
         */
        if ((change == KVM_MR_CREATE) || (change == KVM_MR_MOVE)) {
-               kvm_mmu_zap_all(kvm);
+               kvm_mmu_zap_mmio_sptes(kvm);
                kvm_reload_remote_mmus(kvm);
        }
 }