kvm: check tlbs_dirty directly

In kvm_mmu_notifier_invalidate_range_start(), tlbs_dirty is used as:
        need_tlb_flush |= kvm->tlbs_dirty;
with need_tlb_flush's type being int and tlbs_dirty's type being long.

It means that tlbs_dirty is always used as int and the higher 32 bits
is useless.  We need to check tlbs_dirty in a correct way and this
change checks it directly without propagating it to need_tlb_flush.

Note: it's _extremely_ unlikely this neglecting of higher 32 bits can
cause problems in practice.  It would require encountering tlbs_dirty
on a 4 billion count boundary, and KVM would need to be using shadow
paging or be running a nested guest.

Cc: stable@vger.kernel.org
Fixes: a5201c6bafef ("KVM: MMU: delay flush all tlbs on sync_page path")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20201217154118.16497-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 3abcb2ce5b7db75f2f1963c63433fa8016c679bb..19dae28904f716227549d0be2fb3dfb9aaa02510 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -485,9 +485,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
        kvm->mmu_notifier_count++;
        need_tlb_flush = kvm_unmap_hva_range(kvm, range->start, range->end,
                                             range->flags);
-       need_tlb_flush |= kvm->tlbs_dirty;
        /* we've to flush the tlb before the pages can be freed */
-       if (need_tlb_flush)
+       if (need_tlb_flush || kvm->tlbs_dirty)
                kvm_flush_remote_tlbs(kvm);
 
        spin_unlock(&kvm->mmu_lock);