[PATCH v3 2/5] KVM: Add file back-pointer to struct kvm

[Steffen Eiden <seiden@linux.ibm.com>]

Add kvm->file as a no-reference back-pointer to the VM file in struct
kvm. The pointer is set at VM creation time and cleared under
WRITE_ONCE() in kvm_vm_release() before the file is freed.
Callers storing the file must take their own reference.

Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
---
 include/linux/kvm_host.h | 6 ++++++
 virt/kvm/kvm_main.c      | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index e840ceda9f16..943b108f01cd 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -784,6 +784,12 @@ struct kvm {
 	 * kvm_swap_active_memslots().
 	 */
 	struct mutex slots_arch_lock;
+	/*
+	 * Back-reference to the VM file for subsystems (e.g., VFIO). Holds no
+	 * reference to avoid pinning the VM. Callers storing the file must
+	 * take their own reference.
+	 */
+	struct file *file;
 	struct mm_struct *mm; /* userspace tied to this vm */
 	unsigned long nr_memslot_pages;
 	/* The two memslot sets - active and inactive (per address space) */
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0172ca94f82e..bab71d398236 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1352,6 +1352,8 @@ static int kvm_vm_release(struct inode *inode, struct file *filp)
 
 	kvm_irqfd_release(kvm);
 
+	WRITE_ONCE(kvm->file, NULL);
+
 	kvm_put_kvm(kvm);
 	return 0;
 }
@@ -5532,6 +5534,8 @@ static int kvm_dev_ioctl_create_vm(unsigned long type)
 		goto put_kvm;
 	}
 
+	kvm->file = file;
+
 	/*
 	 * Don't call kvm_put_kvm anymore at this point; file->f_op is
 	 * already set, with ->release() being kvm_vm_release().  In error

-- 
2.53.0