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Commit a355aa54 authored Dec 12, 2011 by Paul Mackerras Committed by Avi Kivity Mar 05, 2012

KVM: Add barriers to allow mmu_notifier_retry to be used locklessly



This adds an smp_wmb in kvm_mmu_notifier_invalidate_range_end() and an
smp_rmb in mmu_notifier_retry() so that mmu_notifier_retry() will give
the correct answer when called without kvm->mmu_lock being held.
PowerPC Book3S HV KVM wants to use a bitlock per guest page rather than
a single global spinlock in order to improve the scalability of updates
to the guest MMU hashed page table, and so needs this.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>

parent 342d3db7

include/linux/kvm_host.h

+9 −5

Original line number	Original line	Diff line number	Diff line
	@@ -702,12 +702,16 @@ static inline int mmu_notifier_retry(struct kvm_vcpu *vcpu, unsigned long mmu_se
	if (unlikely(vcpu->kvm->mmu_notifier_count))		if (unlikely(vcpu->kvm->mmu_notifier_count))
	return 1;		return 1;
	/*		/*
	* Both reads happen under the mmu_lock and both values are		* Ensure the read of mmu_notifier_count happens before the read
	* modified under mmu_lock, so there's no need of smb_rmb()		* of mmu_notifier_seq. This interacts with the smp_wmb() in
	* here in between, otherwise mmu_notifier_count should be		* mmu_notifier_invalidate_range_end to make sure that the caller
	* read before mmu_notifier_seq, see		* either sees the old (non-zero) value of mmu_notifier_count or
	* mmu_notifier_invalidate_range_end write side.		* the new (incremented) value of mmu_notifier_seq.
			* PowerPC Book3s HV KVM calls this under a per-page lock
			* rather than under kvm->mmu_lock, for scalability, so
			* can't rely on kvm->mmu_lock to keep things ordered.
	*/		*/
			smp_rmb();
	if (vcpu->kvm->mmu_notifier_seq != mmu_seq)		if (vcpu->kvm->mmu_notifier_seq != mmu_seq)
	return 1;		return 1;
	return 0;		return 0;

virt/kvm/kvm_main.c

+3 −3

Original line number	Original line	Diff line number	Diff line
	@@ -357,11 +357,11 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
	* been freed.		* been freed.
	*/		*/
	kvm->mmu_notifier_seq++;		kvm->mmu_notifier_seq++;
			smp_wmb();
	/*		/*
	* The above sequence increase must be visible before the		* The above sequence increase must be visible before the
	* below count decrease but both values are read by the kvm		* below count decrease, which is ensured by the smp_wmb above
	* page fault under mmu_lock spinlock so we don't need to add		* in conjunction with the smp_rmb in mmu_notifier_retry().
	* a smb_wmb() here in between the two.
	*/		*/
	kvm->mmu_notifier_count--;		kvm->mmu_notifier_count--;
	spin_unlock(&kvm->mmu_lock);		spin_unlock(&kvm->mmu_lock);