KVM: Add separate helper for putting borrowed reference to kvm

Add a new helper, kvm_put_kvm_no_destroy(), to handle putting a borrowed
reference[*] to the VM when installing a new file descriptor fails.  KVM
expects the refcount to remain valid in this case, as the in-progress
ioctl() has an explicit reference to the VM.  The primary motiviation
for the helper is to document that the 'kvm' pointer is still valid
after putting the borrowed reference, e.g. to document that doing
mutex(&kvm->lock) immediately after putting a ref to kvm isn't broken.

[*] When exposing a new object to userspace via a file descriptor, e.g.
    a new vcpu, KVM grabs a reference to itself (the VM) prior to making
    the object visible to userspace to avoid prematurely freeing the VM
    in the scenario where userspace immediately closes file descriptor.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 2 insertions, 2 deletions

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 9a75f0e1933b..68678e31c84c 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -2000,7 +2000,7 @@ int kvm_vm_ioctl_get_htab_fd(struct kvm *kvm, struct kvm_get_htab_fd *ghf)
 	ret = anon_inode_getfd("kvm-htab", &kvm_htab_fops, ctx, rwflag | O_CLOEXEC);
 	if (ret < 0) {
 		kfree(ctx);
-		kvm_put_kvm(kvm);
+		kvm_put_kvm_no_destroy(kvm);
 		return ret;
 	}
 
diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
index 5834db0a54c6..883a66e76638 100644
--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c
@@ -317,7 +317,7 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 	if (ret >= 0)
 		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
 	else
-		kvm_put_kvm(kvm);
+		kvm_put_kvm_no_destroy(kvm);
 
 	mutex_unlock(&kvm->lock);