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Commit 982b3394 authored by Takuya Yoshikawa's avatar Takuya Yoshikawa Committed by Gleb Natapov
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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: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarTakuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: default avatarGleb Natapov <gleb@redhat.com>
parent 95b0430d

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 @@ void kvm_mmu_zap_all(struct kvm *kvm) 
	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);

	}

}