From: Paul Mackerras <paulus@samba.org>
To: Alexander Graf <agraf@suse.de>
Cc: linuxppc-dev@ozlabs.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org
Subject: [PATCH 11/13] KVM: Add barriers to allow mmu_notifier_retry to be used locklessly
Date: Tue, 6 Dec 2011 17:13:13 +1100 [thread overview]
Message-ID: <20111206061313.GO12389@drongo> (raw)
In-Reply-To: <20111206060156.GD12389@drongo>
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>
---
include/linux/kvm_host.h | 14 +++++++++-----
virt/kvm/kvm_main.c | 6 +++---
2 files changed, 12 insertions(+), 8 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index c6a2ec9..17319ad 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -672,12 +672,16 @@ 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 d9cfb78..ad2a912 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);
--
1.7.5.4
next prev parent reply other threads:[~2011-12-06 6:13 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-06 6:01 [PATCH 0/13] KVM: PPC: Update Book3S HV memory handling Paul Mackerras
2011-12-06 6:02 ` [PATCH 01/13] KVM: PPC: Move kvm_vcpu_ioctl_[gs]et_one_reg down to platform-specific code Paul Mackerras
2011-12-06 6:03 ` [PATCH 02/13] KVM: PPC: Keep a record of HV guest view of hashed page table entries Paul Mackerras
2011-12-06 6:04 ` [PATCH 03/13] KVM: PPC: Keep page physical addresses in per-slot arrays Paul Mackerras
2011-12-06 6:07 ` [PATCH 04/13] KVM: PPC: Add an interface for pinning guest pages in Book3s HV guests Paul Mackerras
2011-12-06 6:08 ` [PATCH 05/13] KVM: PPC: Make the H_ENTER hcall more reliable Paul Mackerras
2011-12-06 6:08 ` [PATCH 06/13] KVM: PPC: Only get pages when actually needed, not in prepare_memory_region() Paul Mackerras
2011-12-06 6:09 ` [PATCH 07/13] KVM: PPC: Allow use of small pages to back Book3S HV guests Paul Mackerras
2011-12-06 6:09 ` [PATCH 08/13] KVM: PPC: Allow I/O mappings in memory slots Paul Mackerras
2011-12-06 6:10 ` [PATCH 09/13] KVM: PPC: Maintain a doubly-linked list of guest HPTEs for each gfn Paul Mackerras
2011-12-06 6:10 ` [PATCH 10/13] KVM: PPC: Implement MMIO emulation support for Book3S HV guests Paul Mackerras
2011-12-06 6:13 ` Paul Mackerras [this message]
2011-12-06 6:14 ` [PATCH 12/13] KVM: PPC: Implement MMU notifiers " Paul Mackerras
2011-12-06 6:14 ` [PATCH 13/13] KVM: PPC: Allow for read-only pages backing a Book3S HV guest Paul Mackerras
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