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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>
This commit is contained in:
Paul Mackerras 2011-12-12 12:37:21 +00:00 committed by Avi Kivity
parent 342d3db763
commit a355aa54f1
2 changed files with 12 additions and 8 deletions
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14
include/linux/kvm_host.h
View File

@ -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;

6
virt/kvm/kvm_main.c
View File

@ -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);