KVM: x86: Optimize mmio spte zapping when creating/moving memslot (982b3394) · Commits · 戴 / test

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>

arch/x86/include/asm/kvm_host.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -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);

arch/x86/kvm/mmu.c

+18 −0

Original line number	Diff line number	Diff line
		@@ -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;

arch/x86/kvm/x86.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -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);
		}
		}

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