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 9b75cae..3f205c6c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -767,6 +767,7 @@
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 de45ec1..c1a9b7b 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4189,6 +4189,24 @@
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 61a5bb6..d3c4787 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -6991,7 +6991,7 @@
* 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);
}
}