[RFC 1/2] vhost-user: Add interface for virtio-fs migration

[Hanna Czenczek]

Add a virtio-fs-specific vhost-user interface to facilitate migrating
back-end-internal state.  We plan to migrate the internal state simply
as a binary blob after the streaming phase, so all we need is a way to
transfer such a blob from and to the back-end.  We do so by using a
dedicated area of shared memory through which the blob is transferred in
chunks.

This patch adds the following vhost operations (and implements them for
vhost-user):

- FS_SET_STATE_FD: The front-end passes a dedicated shared memory area
  to the back-end.  This area will be used to transfer state via the
  other two operations.
  (After the transfer FS_SET_STATE_FD detaches the shared memory area
  again.)

- FS_GET_STATE: The front-end asks the back-end to place a chunk of
  internal state into the shared memory area.

- FS_SET_STATE: The front-end puts a chunk of internal state into the
  shared memory area, and asks the back-end to fetch it.

On the source side, the back-end is expected to serialize its internal
state either when FS_SET_STATE_FD is invoked, or when FS_GET_STATE is
invoked the first time.  On subsequent FS_GET_STATE calls, it memcpy()s
parts of that serialized state into the shared memory area.

On the destination side, the back-end is expected to collect the state
blob over all FS_SET_STATE calls, and then deserialize and apply it once
FS_SET_STATE_FD detaches the shared memory area.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 include/hw/virtio/vhost-backend.h |   9 ++
 include/hw/virtio/vhost.h         |  68 +++++++++++++++
 hw/virtio/vhost-user.c            | 138 ++++++++++++++++++++++++++++++
 hw/virtio/vhost.c                 |  29 +++++++
 4 files changed, 244 insertions(+)

diff --git a/include/hw/virtio/vhost-backend.h 
b/include/hw/virtio/vhost-backend.h
index ec3fbae58d..fa3bd19386 100644
--- a/include/hw/virtio/vhost-backend.h
+++ b/include/hw/virtio/vhost-backend.h
@@ -42,6 +42,12 @@ typedef int (*vhost_backend_init)(struct vhost_dev *dev, 
void *opaque,
 typedef int (*vhost_backend_cleanup)(struct vhost_dev *dev);
 typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
 
+typedef ssize_t (*vhost_fs_get_state_op)(struct vhost_dev *dev,
+                                         uint64_t state_offset, size_t size);
+typedef int (*vhost_fs_set_state_op)(struct vhost_dev *dev,
+                                     uint64_t state_offset, size_t size);
+typedef int (*vhost_fs_set_state_fd_op)(struct vhost_dev *dev, int memfd,
+                                        size_t size);
 typedef int (*vhost_net_set_backend_op)(struct vhost_dev *dev,
                                 struct vhost_vring_file *file);
 typedef int (*vhost_net_set_mtu_op)(struct vhost_dev *dev, uint16_t mtu);
@@ -138,6 +144,9 @@ typedef struct VhostOps {
     vhost_backend_init vhost_backend_init;
     vhost_backend_cleanup vhost_backend_cleanup;
     vhost_backend_memslots_limit vhost_backend_memslots_limit;
+    vhost_fs_get_state_op vhost_fs_get_state;
+    vhost_fs_set_state_op vhost_fs_set_state;
+    vhost_fs_set_state_fd_op vhost_fs_set_state_fd;
     vhost_net_set_backend_op vhost_net_set_backend;
     vhost_net_set_mtu_op vhost_net_set_mtu;
     vhost_scsi_set_endpoint_op vhost_scsi_set_endpoint;
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index a52f273347..b1ad9785dd 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -336,4 +336,72 @@ int vhost_dev_set_inflight(struct vhost_dev *dev,
                            struct vhost_inflight *inflight);
 int vhost_dev_get_inflight(struct vhost_dev *dev, uint16_t queue_size,
                            struct vhost_inflight *inflight);
+
+/**
+ * vhost_fs_set_state_fd(): Share memory with a virtio-fs vhost
+ * back-end for transferring internal state for the purpose of
+ * migration.  Calling this function again will have the back-end
+ * unregister (free) the previously shared memory area.
+ *
+ * @dev: The vhost device
+ * @memfd: File descriptor associated with the shared memory to share.
+ *         If negative, no memory area is shared, only releasing the
+ *         previously shared area, and announcing the end of transfer
+ *         (which, on the destination side, should lead to the
+ *         back-end deserializing and applying the received state).
+ * @size: Size of the shared memory area
+ *
+ * Returns 0 on success, and -errno on failure.
+ */
+int vhost_fs_set_state_fd(struct vhost_dev *dev, int memfd, size_t size);
+
+/**
+ * vhost_fs_get_state(): Request the virtio-fs vhost back-end to place
+ * a chunk of migration state into the shared memory area negotiated
+ * through vhost_fs_set_state_fd().  May only be used for migration,
+ * and only by the source side.
+ *
+ * The back-end-internal migration state is treated as a binary blob,
+ * which is transferred in chunks to fit into the shared memory area.
+ *
+ * @dev: The vhost device
+ * @state_offset: Offset into the state blob of the first byte to be
+ *                transferred
+ * @size: Number of bytes to transfer at most; must not exceed the
+ *        size of the shared memory area
+ *
+ * On success, returns the number of bytes that remain in the full
+ * state blob from the beginning of this chunk (i.e. the full size of
+ * the blob, minus @state_offset).  When transferring the final chunk,
+ * this may be less than @size.  The shared memory will contain the
+ * requested data, starting at offset 0 into the SHM, and counting
+ * `MIN(@size, returned value)` bytes.
+ *
+ * On failure, returns -errno.
+ */
+ssize_t vhost_fs_get_state(struct vhost_dev *dev, uint64_t state_offset,
+                           uint64_t size);
+
+/**
+ * vhost_fs_set_state(): Request the virtio-fs vhost back-end to fetch
+ * a chunk of migration state from the shared memory area negotiated
+ * through vhost_fs_set_state_fd().  May only be used for migration,
+ * and only by the destination side.
+ *
+ * The back-end-internal migration state is treated as a binary blob,
+ * which is transferred in chunks to fit into the shared memory area.
+ *
+ * The front-end (i.e. the caller) must transfer the whole state to
+ * the back-end, without holes.
+ *
+ * @vdev: the VirtIODevice structure
+ * @state_offset: Offset into the state blob of the first byte to be
+ *                transferred
+ * @size: Length of the chunk to transfer; must not exceed the size of
+ *        the shared memory area
+ *
+ * Returns 0 on success, and -errno on failure.
+ */
+int vhost_fs_set_state(struct vhost_dev *dev, uint64_t state_offset,
+                       uint64_t size);
 #endif
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index e5285df4ba..7fd1fb1ed3 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -130,6 +130,9 @@ typedef enum VhostUserRequest {
     VHOST_USER_REM_MEM_REG = 38,
     VHOST_USER_SET_STATUS = 39,
     VHOST_USER_GET_STATUS = 40,
+    VHOST_USER_FS_SET_STATE_FD = 41,
+    VHOST_USER_FS_GET_STATE = 42,
+    VHOST_USER_FS_SET_STATE = 43,
     VHOST_USER_MAX
 } VhostUserRequest;
 
@@ -210,6 +213,15 @@ typedef struct {
     uint32_t size; /* the following payload size */
 } QEMU_PACKED VhostUserHeader;
 
+/*
+ * Request and reply payloads of VHOST_USER_FS_GET_STATE, and request
+ * payload of VHOST_USER_FS_SET_STATE.
+ */
+typedef struct VhostUserFsState {
+    uint64_t state_offset;
+    uint64_t size;
+} VhostUserFsState;
+
 typedef union {
 #define VHOST_USER_VRING_IDX_MASK   (0xff)
 #define VHOST_USER_VRING_NOFD_MASK  (0x1 << 8)
@@ -224,6 +236,7 @@ typedef union {
         VhostUserCryptoSession session;
         VhostUserVringArea area;
         VhostUserInflight inflight;
+        VhostUserFsState fs_state;
 } VhostUserPayload;
 
 typedef struct VhostUserMsg {
@@ -2240,6 +2253,128 @@ static int vhost_user_net_set_mtu(struct vhost_dev 
*dev, uint16_t mtu)
     return 0;
 }
 
+static int vhost_user_fs_set_state_fd(struct vhost_dev *dev, int memfd,
+                                      size_t size)
+{
+    int ret;
+    bool reply_supported = virtio_has_feature(dev->protocol_features,
+                                              VHOST_USER_PROTOCOL_F_REPLY_ACK);
+    VhostUserMsg msg = {
+        .hdr = {
+            .request = VHOST_USER_FS_SET_STATE_FD,
+            .flags = VHOST_USER_VERSION,
+            .size = sizeof(msg.payload.u64),
+        },
+        .payload.u64 = size,
+    };
+
+    if (reply_supported) {
+        msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
+    }
+
+    if (memfd < 0) {
+        assert(size == 0);
+        ret = vhost_user_write(dev, &msg, NULL, 0);
+    } else {
+        ret = vhost_user_write(dev, &msg, &memfd, 1);
+    }
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (reply_supported) {
+        return process_message_reply(dev, &msg);
+    }
+
+    return 0;
+}
+
+static ssize_t vhost_user_fs_get_state(struct vhost_dev *dev,
+                                       uint64_t state_offset,
+                                       size_t size)
+{
+    int ret;
+    VhostUserMsg msg = {
+        .hdr = {
+            .request = VHOST_USER_FS_GET_STATE,
+            .flags = VHOST_USER_VERSION,
+            .size = sizeof(msg.payload.fs_state),
+        },
+        .payload.fs_state = {
+            .state_offset = state_offset,
+            .size = size,
+        },
+    };
+
+    ret = vhost_user_write(dev, &msg, NULL, 0);
+    if (ret < 0) {
+        return ret;
+    }
+
+    ret = vhost_user_read(dev, &msg);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (msg.hdr.request != VHOST_USER_FS_GET_STATE) {
+        error_report("Received unexpected message type: "
+                     "Expected %d, received %d",
+                     VHOST_USER_FS_GET_STATE, msg.hdr.request);
+        return -EPROTO;
+    }
+
+    if (msg.hdr.size != sizeof(VhostUserFsState)) {
+        error_report("Received unexpected message length: "
+                     "Expected %" PRIu32 ", received %zu",
+                     msg.hdr.size, sizeof(VhostUserFsState));
+        return -EPROTO;
+    }
+
+    if (msg.payload.fs_state.size > SSIZE_MAX) {
+        error_report("Remaining state size returned by back end is too high: "
+                     "Expected up to %zd, reported %" PRIu64,
+                     SSIZE_MAX, msg.payload.fs_state.size);
+        return -EPROTO;
+    }
+
+    return msg.payload.fs_state.size;
+}
+
+static int vhost_user_fs_set_state(struct vhost_dev *dev,
+                                   uint64_t state_offset,
+                                   size_t size)
+{
+    int ret;
+    bool reply_supported = virtio_has_feature(dev->protocol_features,
+                                              VHOST_USER_PROTOCOL_F_REPLY_ACK);
+    VhostUserMsg msg = {
+        .hdr = {
+            .request = VHOST_USER_FS_SET_STATE,
+            .flags = VHOST_USER_VERSION,
+            .size = sizeof(msg.payload.fs_state),
+        },
+        .payload.fs_state = {
+            .state_offset = state_offset,
+            .size = size,
+        },
+    };
+
+    if (reply_supported) {
+        msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
+    }
+
+    ret = vhost_user_write(dev, &msg, NULL, 0);
+    if (ret < 0) {
+        return ret;
+    }
+
+    if (reply_supported) {
+        return process_message_reply(dev, &msg);
+    }
+
+    return 0;
+}
+
 static int vhost_user_send_device_iotlb_msg(struct vhost_dev *dev,
                                             struct vhost_iotlb_msg *imsg)
 {
@@ -2716,4 +2851,7 @@ const VhostOps user_ops = {
         .vhost_get_inflight_fd = vhost_user_get_inflight_fd,
         .vhost_set_inflight_fd = vhost_user_set_inflight_fd,
         .vhost_dev_start = vhost_user_dev_start,
+        .vhost_fs_set_state_fd = vhost_user_fs_set_state_fd,
+        .vhost_fs_get_state = vhost_user_fs_get_state,
+        .vhost_fs_set_state = vhost_user_fs_set_state,
 };
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index a266396576..ef8252c90e 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -2075,3 +2075,32 @@ int vhost_net_set_backend(struct vhost_dev *hdev,
 
     return -ENOSYS;
 }
+
+int vhost_fs_set_state_fd(struct vhost_dev *dev, int memfd, size_t size)
+{
+    if (dev->vhost_ops->vhost_fs_set_state_fd) {
+        return dev->vhost_ops->vhost_fs_set_state_fd(dev, memfd, size);
+    }
+
+    return -ENOSYS;
+}
+
+ssize_t vhost_fs_get_state(struct vhost_dev *dev, uint64_t state_offset,
+                           uint64_t size)
+{
+    if (dev->vhost_ops->vhost_fs_get_state) {
+        return dev->vhost_ops->vhost_fs_get_state(dev, state_offset, size);
+    }
+
+    return -ENOSYS;
+}
+
+int vhost_fs_set_state(struct vhost_dev *dev, uint64_t state_offset,
+                       uint64_t size)
+{
+    if (dev->vhost_ops->vhost_fs_set_state) {
+        return dev->vhost_ops->vhost_fs_set_state(dev, state_offset, size);
+    }
+
+    return -ENOSYS;
+}
-- 
2.39.1