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>

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d4d4d70..eada8e6 100644 (file)
--- 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 (file)
--- 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);