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/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);
}
}