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KVM: Fix write protection race during dirty logging

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This patch fixes a race introduced by:

  commit 95d4c16ce7
  KVM: Optimize dirty logging by rmap_write_protect()

During protecting pages for dirty logging, other threads may also try
to protect a page in mmu_sync_children() or kvm_mmu_get_page().

In such a case, because get_dirty_log releases mmu_lock before flushing
TLB's, the following race condition can happen:

  A (get_dirty_log)     B (another thread)

  lock(mmu_lock)
  clear pte.w
  unlock(mmu_lock)
                        lock(mmu_lock)
                        pte.w is already cleared
                        unlock(mmu_lock)
                        skip TLB flush
                        return
  ...
  TLB flush

Though thread B assumes the page has already been protected when it
returns, the remaining TLB entry will break that assumption.

This patch fixes this problem by making get_dirty_log hold the mmu_lock
until it flushes the TLB's.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Takuya Yoshikawa 2012-02-05 20:42:41 +09:00 committed by Avi Kivity
parent 10166744b8
commit 6dbf79e716
1 changed files with 5 additions and 6 deletions
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11
arch/x86/kvm/x86.c
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@ -3065,6 +3065,8 @@ static void write_protect_slot(struct kvm *kvm,
unsigned long *dirty_bitmap,
unsigned long nr_dirty_pages)
{
spin_lock(&kvm->mmu_lock);
/* Not many dirty pages compared to # of shadow pages. */
if (nr_dirty_pages < kvm->arch.n_used_mmu_pages) {
unsigned long gfn_offset;
@ -3072,16 +3074,13 @@ static void write_protect_slot(struct kvm *kvm,
for_each_set_bit(gfn_offset, dirty_bitmap, memslot->npages) {
unsigned long gfn = memslot->base_gfn + gfn_offset;
spin_lock(&kvm->mmu_lock);
kvm_mmu_rmap_write_protect(kvm, gfn, memslot);
spin_unlock(&kvm->mmu_lock);
}
kvm_flush_remote_tlbs(kvm);
} else {
spin_lock(&kvm->mmu_lock);
} else
kvm_mmu_slot_remove_write_access(kvm, memslot->id);
spin_unlock(&kvm->mmu_lock);
}
spin_unlock(&kvm->mmu_lock);
}
/*