From a355aa54f1d25dff83c0feef8863d83a76988fdb Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Mon, 12 Dec 2011 12:37:21 +0000 Subject: 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 Acked-by: Avi Kivity Signed-off-by: Alexander Graf Signed-off-by: Avi Kivity diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index d4d4d70..eada8e6 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -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)) return 1; /* - * Both reads happen under the mmu_lock and both values are - * modified under mmu_lock, so there's no need of smb_rmb() - * here in between, otherwise mmu_notifier_count should be - * read before mmu_notifier_seq, see - * mmu_notifier_invalidate_range_end write side. + * Ensure the read of mmu_notifier_count happens before the read + * of mmu_notifier_seq. This interacts with the smp_wmb() in + * mmu_notifier_invalidate_range_end to make sure that the caller + * either sees the old (non-zero) value of mmu_notifier_count or + * 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) return 1; return 0; diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 64be836..9f32bff 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -357,11 +357,11 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn, * been freed. */ kvm->mmu_notifier_seq++; + smp_wmb(); /* * The above sequence increase must be visible before the - * below count decrease but both values are read by the kvm - * page fault under mmu_lock spinlock so we don't need to add - * a smb_wmb() here in between the two. + * below count decrease, which is ensured by the smp_wmb above + * in conjunction with the smp_rmb in mmu_notifier_retry(). */ kvm->mmu_notifier_count--; spin_unlock(&kvm->mmu_lock); -- cgit v0.10.2