[PATCH V2 3/7] vhost-vdpa: VHOST_NEW_OWNER

[Steve Sistare]

Add an ioctl to transfer file descriptor ownership and pinned memory
accounting from one process to another.

This is more efficient than VHOST_RESET_OWNER followed by VHOST_SET_OWNER,
as that would unpin all physical pages, requiring them to be repinned in
the new process.  That would cost multiple seconds for large memories, and
be incurred during a virtual machine's pause time during live update.

Signed-off-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
---
 drivers/vhost/vdpa.c       | 41 ++++++++++++++++++++++++++++++++++++++
 drivers/vhost/vhost.c      | 15 ++++++++++++++
 drivers/vhost/vhost.h      |  1 +
 include/uapi/linux/vhost.h | 10 ++++++++++
 4 files changed, 67 insertions(+)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index b49e5831b3f0..5cf55ca4ec02 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -632,6 +632,44 @@ static long vhost_vdpa_resume(struct vhost_vdpa *v)
 	return ret;
 }
 
+static long vhost_vdpa_new_owner(struct vhost_vdpa *v)
+{
+	int r;
+	struct vhost_dev *vdev = &v->vdev;
+	struct mm_struct *mm_old = vdev->mm;
+	struct mm_struct *mm_new = current->mm;
+	long pinned_vm = v->pinned_vm;
+	unsigned long lock_limit = PFN_DOWN(rlimit(RLIMIT_MEMLOCK));
+
+	if (!mm_old)
+		return -EINVAL;
+	mmgrab(mm_old);
+
+	if (!v->vdpa->use_va &&
+	    pinned_vm + atomic64_read(&mm_new->pinned_vm) > lock_limit) {
+		r = -ENOMEM;
+		goto out;
+	}
+	r = vhost_vdpa_bind_mm(v, mm_new);
+	if (r)
+		goto out;
+
+	r = vhost_dev_new_owner(vdev);
+	if (r) {
+		vhost_vdpa_bind_mm(v, mm_old);
+		goto out;
+	}
+
+	if (!v->vdpa->use_va) {
+		atomic64_sub(pinned_vm, &mm_old->pinned_vm);
+		atomic64_add(pinned_vm, &mm_new->pinned_vm);
+	}
+
+out:
+	mmdrop(mm_old);
+	return r;
+}
+
 static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
 				   void __user *argp)
 {
@@ -876,6 +914,9 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 	case VHOST_VDPA_RESUME:
 		r = vhost_vdpa_resume(v);
 		break;
+	case VHOST_NEW_OWNER:
+		r = vhost_vdpa_new_owner(v);
+		break;
 	default:
 		r = vhost_dev_ioctl(&v->vdev, cmd, argp);
 		if (r == -ENOIOCTLCMD)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index b60955682474..ab40ae50552f 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -963,6 +963,21 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_set_owner);
 
+/* Caller should have device mutex */
+long vhost_dev_new_owner(struct vhost_dev *dev)
+{
+	if (dev->mm == current->mm)
+		return -EBUSY;
+
+	if (!vhost_dev_has_owner(dev))
+		return -EINVAL;
+
+	vhost_detach_mm(dev);
+	vhost_attach_mm(dev);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(vhost_dev_new_owner);
+
 static struct vhost_iotlb *iotlb_alloc(void)
 {
 	return vhost_iotlb_alloc(max_iotlb_entries,
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index bb75a292d50c..8b2018bb02b1 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -187,6 +187,7 @@ void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs,
 		    int (*msg_handler)(struct vhost_dev *dev, u32 asid,
 				       struct vhost_iotlb_msg *msg));
 long vhost_dev_set_owner(struct vhost_dev *dev);
+long vhost_dev_new_owner(struct vhost_dev *dev);
 bool vhost_dev_has_owner(struct vhost_dev *dev);
 long vhost_dev_check_owner(struct vhost_dev *);
 struct vhost_iotlb *vhost_dev_reset_owner_prepare(void);
diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index b95dd84eef2d..543d0e3434c3 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -123,6 +123,16 @@
 #define VHOST_SET_BACKEND_FEATURES _IOW(VHOST_VIRTIO, 0x25, __u64)
 #define VHOST_GET_BACKEND_FEATURES _IOR(VHOST_VIRTIO, 0x26, __u64)
 
+/* Set current process as the new owner of this file descriptor.  The fd must
+ * already be owned, via a prior call to VHOST_SET_OWNER.  The pinned memory
+ * count is transferred from the previous to the new owner.
+ * Errors:
+ *   EINVAL: not owned
+ *   EBUSY:  caller is already the owner
+ *   ENOMEM: RLIMIT_MEMLOCK exceeded
+ */
+#define VHOST_NEW_OWNER _IO(VHOST_VIRTIO, 0x27)
+
 /* VHOST_NET specific defines */
 
 /* Attach virtio net ring to a raw socket, or tap device.
-- 
2.39.3