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authorPaul Mackerras <paulus@samba.org>2011-12-12 12:37:21 +0000
committerAvi Kivity <avi@redhat.com>2012-03-05 14:52:38 +0200
commita355aa54f1d25dff83c0feef8863d83a76988fdb (patch)
tree4fe27e8dd1a3935757b319726a8b8bdc10496930 /include/linux/kvm_host.h
parent342d3db763f2621ed4546ebf8f6c61cb29d7fbdb (diff)
downloadop-kernel-dev-a355aa54f1d25dff83c0feef8863d83a76988fdb.zip
op-kernel-dev-a355aa54f1d25dff83c0feef8863d83a76988fdb.tar.gz
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>
Diffstat (limited to 'include/linux/kvm_host.h')
-rw-r--r--include/linux/kvm_host.h14
1 files changed, 9 insertions, 5 deletions
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;
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