[Virtio-fs] [PATCH v5 0/7] vhost-user: Back-end state migration

[Virtio-fs] [PATCH v5 0/7] vhost-user: Back-end state migration

[Hanna Czenczek]

v2:
https://lists.nongnu.org/archive/html/qemu-devel/2023-07/msg02604.html

v3:
https://lists.nongnu.org/archive/html/qemu-devel/2023-09/msg03750.html

v4:
https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg01046.html


Based-on: <20231004014532.1228637-1-stefanha@redhat.com>
          ([PATCH v2 0/3] vhost: clean up device reset)

Based-on: <20231016083201.23736-1-hreitz@redhat.com>
          ([PATCH] vhost-user: Fix protocol feature bit conflict)


Hi,

v5 is basically the same as v4, only that I’ve dropped the patch
deprecating F_STATUS (which doesn’t affect the rest of the series), that
I’ve amended the documentation in patch 1 as suggested by Stefan and
with help from Michael, and that I’ve rebased everything on top of the
F_SHARED_OBJECT changes that have been merged upstream.


git-backport-diff against v4:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/7:[0029] [FC] 'vhost-user.rst: Improve [GS]ET_VRING_BASE doc'
002/7:[----] [--] 'vhost-user.rst: Clarify enabling/disabling vrings'
003/7:[----] [--] 'vhost-user.rst: Introduce suspended state'
004/7:[0006] [FC] 'vhost-user.rst: Migrating back-end-internal state'
005/7:[0007] [FC] 'vhost-user: Interface for migration state transfer'
006/7:[----] [--] 'vhost: Add high-level state save/load functions'
007/7:[----] [--] 'vhost-user-fs: Implement internal migration'


Changes patch by patch:
- Patch 1: Amended documentation
- Patches 4 and 5: Bumped feature bit and command values as necessary so
  as not to conflict with F_SHARED_OBJECT


Hanna Czenczek (7):
  vhost-user.rst: Improve [GS]ET_VRING_BASE doc
  vhost-user.rst: Clarify enabling/disabling vrings
  vhost-user.rst: Introduce suspended state
  vhost-user.rst: Migrating back-end-internal state
  vhost-user: Interface for migration state transfer
  vhost: Add high-level state save/load functions
  vhost-user-fs: Implement internal migration

 docs/interop/vhost-user.rst       | 301 ++++++++++++++++++++++++++++--
 include/hw/virtio/vhost-backend.h |  24 +++
 include/hw/virtio/vhost-user.h    |   1 +
 include/hw/virtio/vhost.h         | 113 +++++++++++
 hw/virtio/vhost-user-fs.c         | 101 +++++++++-
 hw/virtio/vhost-user.c            | 146 +++++++++++++++
 hw/virtio/vhost.c                 | 241 ++++++++++++++++++++++++
 7 files changed, 906 insertions(+), 21 deletions(-)

-- 
2.41.0

[Virtio-fs] [PATCH v5 1/7] vhost-user.rst: Improve [GS]ET_VRING_BASE doc

[Hanna Czenczek]

GET_VRING_BASE does not mention that it stops the respective ring.  Fix
that.

Furthermore, it is not fully clear what the "base offset" these
commands' documentation refers to is; an offset could be many things.
Be more precise and verbose about it, especially given that these
commands use different payload structures depending on whether the vring
is split or packed.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 docs/interop/vhost-user.rst | 77 +++++++++++++++++++++++++++++++++++--
 1 file changed, 73 insertions(+), 4 deletions(-)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 768fb5c28c..9202b167dd 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -108,6 +108,43 @@ A vring state description
 
 :num: a 32-bit number
 
+A vring descriptor index for split virtqueues
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++-------------+---------------------+
+| vring index | index in avail ring |
++-------------+---------------------+
+
+:vring index: 32-bit index of the respective virtqueue
+
+:index in avail ring: 32-bit value, of which currently only the lower 16
+  bits are used:
+
+  - Bits 0–15: Index of the next *Available Ring* descriptor that the
+    back-end will process.  This is a free-running index that is not
+    wrapped by the ring size.
+  - Bits 16–31: Reserved (set to zero)
+
+Vring descriptor indices for packed virtqueues
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++-------------+--------------------+
+| vring index | descriptor indices |
++-------------+--------------------+
+
+:vring index: 32-bit index of the respective virtqueue
+
+:descriptor indices: 32-bit value:
+
+  - Bits 0–14: Index of the next *Available Ring* descriptor that the
+    back-end will process.  This is a free-running index that is not
+    wrapped by the ring size.
+  - Bit 15: Driver (Available) Ring Wrap Counter
+  - Bits 16–30: Index of the entry in the *Used Ring* where the back-end
+    will place the next descriptor.  This is a free-running index that
+    is not wrapped by the ring size.
+  - Bit 31: Device (Used) Ring Wrap Counter
+
 A vring address description
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1042,18 +1079,50 @@ Front-end message types
 ``VHOST_USER_SET_VRING_BASE``
   :id: 10
   :equivalent ioctl: ``VHOST_SET_VRING_BASE``
-  :request payload: vring state description
+  :request payload: vring descriptor index/indices
   :reply payload: N/A
 
-  Sets the base offset in the available vring.
+  Sets the next index to use for descriptors in this vring:
+
+  * For a split virtqueue, sets only the next descriptor index to
+    process in the *Available Ring*.  The device is supposed to read the
+    next index in the *Used Ring* from the respective vring structure in
+    guest memory.
+
+  * For a packed virtqueue, both indices are supplied, as they are not
+    explicitly available in memory.
+
+  Consequently, the payload type is specific to the type of virt queue
+  (*a vring descriptor index for split virtqueues* vs. *vring descriptor
+  indices for packed virtqueues*).
 
 ``VHOST_USER_GET_VRING_BASE``
   :id: 11
   :equivalent ioctl: ``VHOST_USER_GET_VRING_BASE``
   :request payload: vring state description
-  :reply payload: vring state description
+  :reply payload: vring descriptor index/indices
+
+  Stops the vring and returns the current descriptor index or indices:
+
+    * For a split virtqueue, returns only the 16-bit next descriptor
+      index to process in the *Available Ring*.  Note that this may
+      differ from the available ring index in the vring structure in
+      memory, which points to where the driver will put new available
+      descriptors.  For the *Used Ring*, the device only needs the next
+      descriptor index at which to put new descriptors, which is the
+      value in the vring structure in memory, so this value is not
+      covered by this message.
+
+    * For a packed virtqueue, neither index is explicitly available to
+      read from memory, so both indices (as maintained by the device) are
+      returned.
+
+  Consequently, the payload type is specific to the type of virt queue
+  (*a vring descriptor index for split virtqueues* vs. *vring descriptor
+  indices for packed virtqueues*).
 
-  Get the available vring base offset.
+  The request payload’s *num* field is currently reserved and must be
+  set to 0.
 
 ``VHOST_USER_SET_VRING_KICK``
   :id: 12
-- 
2.41.0

[Virtio-fs] [PATCH v5 2/7] vhost-user.rst: Clarify enabling/disabling vrings

[Hanna Czenczek]

Currently, the vhost-user documentation says that rings are to be
initialized in a disabled state when VHOST_USER_F_PROTOCOL_FEATURES is
negotiated.  However, by the time of feature negotiation, all rings have
already been initialized, so it is not entirely clear what this means.

At least the vhost-user-backend Rust crate's implementation interpreted
it to mean that whenever this feature is negotiated, all rings are to
put into a disabled state, which means that every SET_FEATURES call
would disable all rings, effectively halting the device.  This is
problematic because the VHOST_F_LOG_ALL feature is also set or cleared
this way, which happens during migration.  Doing so should not halt the
device.

Other implementations have interpreted this to mean that the device is
to be initialized with all rings disabled, and a subsequent SET_FEATURES
call that does not set VHOST_USER_F_PROTOCOL_FEATURES will enable all of
them.  Here, SET_FEATURES will never disable any ring.

This interpretation does not suffer the problem of unintentionally
halting the device whenever features are set or cleared, so it seems
better and more reasonable.

We can clarify this in the documentation by making it explicit that the
enabled/disabled state is tracked even while the vring is stopped.
Every vring is initialized in a disabled state, and SET_FEATURES without
VHOST_USER_F_PROTOCOL_FEATURES simply becomes one way to enable all
vrings.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 docs/interop/vhost-user.rst | 32 +++++++++++++++++---------------
 1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 9202b167dd..e5a04c04ed 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -411,31 +411,33 @@ negotiation.
 Ring states
 -----------
 
-Rings can be in one of three states:
+Rings have two independent states: started/stopped, and enabled/disabled.
 
-* stopped: the back-end must not process the ring at all.
+* While a ring is stopped, the back-end must not process the ring at
+  all, regardless of whether it is enabled or disabled.  The
+  enabled/disabled state should still be tracked, though, so it can come
+  into effect once the ring is started.
 
-* started but disabled: the back-end must process the ring without
+* started and disabled: The back-end must process the ring without
   causing any side effects.  For example, for a networking device,
   in the disabled state the back-end must not supply any new RX packets,
   but must process and discard any TX packets.
 
-* started and enabled.
+* started and enabled: The back-end must process the ring normally, i.e.
+  process all requests and execute them.
 
-Each ring is initialized in a stopped state.  The back-end must start
-ring upon receiving a kick (that is, detecting that file descriptor is
-readable) on the descriptor specified by ``VHOST_USER_SET_VRING_KICK``
-or receiving the in-band message ``VHOST_USER_VRING_KICK`` if negotiated,
-and stop ring upon receiving ``VHOST_USER_GET_VRING_BASE``.
+Each ring is initialized in a stopped and disabled state.  The back-end
+must start a ring upon receiving a kick (that is, detecting that file
+descriptor is readable) on the descriptor specified by
+``VHOST_USER_SET_VRING_KICK`` or receiving the in-band message
+``VHOST_USER_VRING_KICK`` if negotiated, and stop a ring upon receiving
+``VHOST_USER_GET_VRING_BASE``.
 
 Rings can be enabled or disabled by ``VHOST_USER_SET_VRING_ENABLE``.
 
-If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
-ring starts directly in the enabled state.
-
-If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
-initialized in a disabled state and is enabled by
-``VHOST_USER_SET_VRING_ENABLE`` with parameter 1.
+In addition, upon receiving a ``VHOST_USER_SET_FEATURES`` message from
+the front-end without ``VHOST_USER_F_PROTOCOL_FEATURES`` set, the
+back-end must enable all rings immediately.
 
 While processing the rings (whether they are enabled or not), the back-end
 must support changing some configuration aspects on the fly.
-- 
2.41.0

[Virtio-fs] [PATCH v5 3/7] vhost-user.rst: Introduce suspended state

[Hanna Czenczek]

In vDPA, GET_VRING_BASE does not stop the queried vring, which is why
SUSPEND was introduced so that the returned index would be stable.  In
vhost-user, it does stop the vring, so under the same reasoning, it can
get away without SUSPEND.

Still, we do want to clarify that if the device is completely stopped,
i.e. all vrings are stopped, the back-end should cease to modify any
state relating to the guest.  Do this by calling it "suspended".

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 docs/interop/vhost-user.rst | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index e5a04c04ed..035a23ed35 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -442,6 +442,19 @@ back-end must enable all rings immediately.
 While processing the rings (whether they are enabled or not), the back-end
 must support changing some configuration aspects on the fly.
 
+.. _suspended_device_state:
+
+Suspended device state
+^^^^^^^^^^^^^^^^^^^^^^
+
+While all vrings are stopped, the device is *suspended*.  In addition to
+not processing any vring (because they are stopped), the device must:
+
+* not write to any guest memory regions,
+* not send any notifications to the guest,
+* not send any messages to the front-end,
+* still process and reply to messages from the front-end.
+
 Multiple queue support
 ----------------------
 
@@ -529,7 +542,8 @@ ancillary data, it may be used to inform the front-end that the log has
 been modified.
 
 Once the source has finished migration, rings will be stopped by the
-source. No further update must be done before rings are restarted.
+source (:ref:`Suspended device state <suspended_device_state>`). No
+further update must be done before rings are restarted.
 
 In postcopy migration the back-end is started before all the memory has
 been received from the source host, and care must be taken to avoid
@@ -1123,6 +1137,10 @@ Front-end message types
   (*a vring descriptor index for split virtqueues* vs. *vring descriptor
   indices for packed virtqueues*).
 
+  When and as long as all of a device’s vrings are stopped, it is
+  *suspended*, see :ref:`Suspended device state
+  <suspended_device_state>`.
+
   The request payload’s *num* field is currently reserved and must be
   set to 0.
 
-- 
2.41.0

[Virtio-fs] [PATCH v5 4/7] vhost-user.rst: Migrating back-end-internal state

[Hanna Czenczek]

For vhost-user devices, qemu can migrate the virtio state, but not the
back-end's internal state.  To do so, we need to be able to transfer
this internal state between front-end (qemu) and back-end.

At this point, this new feature is added for the purpose of virtio-fs
migration.  Because virtiofsd's internal state will not be too large, we
believe it is best to transfer it as a single binary blob after the
streaming phase.

These are the additions to the protocol:
- New vhost-user protocol feature VHOST_USER_PROTOCOL_F_DEVICE_STATE
- SET_DEVICE_STATE_FD function: Front-end and back-end negotiate a file
  descriptor over which to transfer the state.
- CHECK_DEVICE_STATE: After the state has been transferred through the
  file descriptor, the front-end invokes this function to verify
  success.  There is no in-band way (through the file descriptor) to
  indicate failure, so we need to check explicitly.

Once the transfer FD has been established via SET_DEVICE_STATE_FD
(which includes establishing the direction of transfer and migration
phase), the sending side writes its data into it, and the reading side
reads it until it sees an EOF.  Then, the front-end will check for
success via CHECK_DEVICE_STATE, which on the destination side includes
checking for integrity (i.e. errors during deserialization).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 docs/interop/vhost-user.rst | 172 ++++++++++++++++++++++++++++++++++++
 1 file changed, 172 insertions(+)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 035a23ed35..9f1103f85a 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -322,6 +322,32 @@ VhostUserShared
 :UUID: 16 bytes UUID, whose first three components (a 32-bit value, then
   two 16-bit values) are stored in big endian.
 
+Device state transfer parameters
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
++--------------------+-----------------+
+| transfer direction | migration phase |
++--------------------+-----------------+
+
+:transfer direction: a 32-bit enum, describing the direction in which
+  the state is transferred:
+
+  - 0: Save: Transfer the state from the back-end to the front-end,
+    which happens on the source side of migration
+  - 1: Load: Transfer the state from the front-end to the back-end,
+    which happens on the destination side of migration
+
+:migration phase: a 32-bit enum, describing the state in which the VM
+  guest and devices are:
+
+  - 0: Stopped (in the period after the transfer of memory-mapped
+    regions before switch-over to the destination): The VM guest is
+    stopped, and the vhost-user device is suspended (see
+    :ref:`Suspended device state <suspended_device_state>`).
+
+  In the future, additional phases might be added e.g. to allow
+  iterative migration while the device is running.
+
 C structure
 -----------
 
@@ -381,6 +407,7 @@ in the ancillary data:
 * ``VHOST_USER_SET_VRING_ERR``
 * ``VHOST_USER_SET_BACKEND_REQ_FD`` (previous name ``VHOST_USER_SET_SLAVE_REQ_FD``)
 * ``VHOST_USER_SET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
+* ``VHOST_USER_SET_DEVICE_STATE_FD``
 
 If *front-end* is unable to send the full message or receives a wrong
 reply it will close the connection. An optional reconnection mechanism
@@ -555,6 +582,80 @@ it performs WAKE ioctl's on the userfaultfd to wake the stalled
 back-end.  The front-end indicates support for this via the
 ``VHOST_USER_PROTOCOL_F_PAGEFAULT`` feature.
 
+.. _migrating_backend_state:
+
+Migrating back-end state
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Migrating device state involves transferring the state from one
+back-end, called the source, to another back-end, called the
+destination.  After migration, the destination transparently resumes
+operation without requiring the driver to re-initialize the device at
+the VIRTIO level.  If the migration fails, then the source can
+transparently resume operation until another migration attempt is made.
+
+Generally, the front-end is connected to a virtual machine guest (which
+contains the driver), which has its own state to transfer between source
+and destination, and therefore will have an implementation-specific
+mechanism to do so.  The ``VHOST_USER_PROTOCOL_F_DEVICE_STATE`` feature
+provides functionality to have the front-end include the back-end's
+state in this transfer operation so the back-end does not need to
+implement its own mechanism, and so the virtual machine may have its
+complete state, including vhost-user devices' states, contained within a
+single stream of data.
+
+To do this, the back-end state is transferred from back-end to front-end
+on the source side, and vice versa on the destination side.  This
+transfer happens over a channel that is negotiated using the
+``VHOST_USER_SET_DEVICE_STATE_FD`` message.  This message has two
+parameters:
+
+* Direction of transfer: On the source, the data is saved, transferring
+  it from the back-end to the front-end.  On the destination, the data
+  is loaded, transferring it from the front-end to the back-end.
+
+* Migration phase: Currently, the only supported phase is the period
+  after the transfer of memory-mapped regions before switch-over to the
+  destination, when both the source and destination devices are
+  suspended (:ref:`Suspended device state <suspended_device_state>`).
+  In the future, additional phases might be supported to allow iterative
+  migration while the device is running.
+
+The nature of the channel is implementation-defined, but it must
+generally behave like a pipe: The writing end will write all the data it
+has into it, signalling the end of data by closing its end.  The reading
+end must read all of this data (until encountering the end of file) and
+process it.
+
+* When saving, the writing end is the source back-end, and the reading
+  end is the source front-end.  After reading the state data from the
+  channel, the source front-end must transfer it to the destination
+  front-end through an implementation-defined mechanism.
+
+* When loading, the writing end is the destination front-end, and the
+  reading end is the destination back-end.  After reading the state data
+  from the channel, the destination back-end must deserialize its
+  internal state from that data and set itself up to allow the driver to
+  seamlessly resume operation on the VIRTIO level.
+
+Seamlessly resuming operation means that the migration must be
+transparent to the guest driver, which operates on the VIRTIO level.
+This driver will not perform any re-initialization steps, but continue
+to use the device as if no migration had occurred.  The vhost-user
+front-end, however, will re-initialize the vhost state on the
+destination, following the usual protocol for establishing a connection
+to a vhost-user back-end: This includes, for example, setting up memory
+mappings and kick and call FDs as necessary, negotiating protocol
+features, or setting the initial vring base indices (to the same value
+as on the source side, so that operation can resume).
+
+Both on the source and on the destination side, after the respective
+front-end has seen all data transferred (when the transfer FD has been
+closed), it sends the ``VHOST_USER_CHECK_DEVICE_STATE`` message to
+verify that data transfer was successful in the back-end, too.  The
+back-end responds once it knows whether the transfer and processing was
+successful or not.
+
 Memory access
 -------------
 
@@ -949,6 +1050,7 @@ Protocol features
   #define VHOST_USER_PROTOCOL_F_STATUS               16
   #define VHOST_USER_PROTOCOL_F_XEN_MMAP             17
   #define VHOST_USER_PROTOCOL_F_SHARED_OBJECT        18
+  #define VHOST_USER_PROTOCOL_F_DEVICE_STATE         19
 
 Front-end message types
 -----------------------
@@ -1553,6 +1655,76 @@ Front-end message types
   the requested UUID. Back-end will reply passing the fd when the operation
   is successful, or no fd otherwise.
 
+``VHOST_USER_SET_DEVICE_STATE_FD``
+  :id: 42
+  :equivalent ioctl: N/A
+  :request payload: device state transfer parameters
+  :reply payload: ``u64``
+
+  Front-end and back-end negotiate a channel over which to transfer the
+  back-end’s internal state during migration.  Either side (front-end or
+  back-end) may create the channel.  The nature of this channel is not
+  restricted or defined in this document, but whichever side creates it
+  must create a file descriptor that is provided to the respectively
+  other side, allowing access to the channel.  This FD must behave as
+  follows:
+
+  * For the writing end, it must allow writing the whole back-end state
+    sequentially.  Closing the file descriptor signals the end of
+    transfer.
+
+  * For the reading end, it must allow reading the whole back-end state
+    sequentially.  The end of file signals the end of the transfer.
+
+  For example, the channel may be a pipe, in which case the two ends of
+  the pipe fulfill these requirements respectively.
+
+  Initially, the front-end creates a channel along with such an FD.  It
+  passes the FD to the back-end as ancillary data of a
+  ``VHOST_USER_SET_DEVICE_STATE_FD`` message.  The back-end may create a
+  different transfer channel, passing the respective FD back to the
+  front-end as ancillary data of the reply.  If so, the front-end must
+  then discard its channel and use the one provided by the back-end.
+
+  Whether the back-end should decide to use its own channel is decided
+  based on efficiency: If the channel is a pipe, both ends will most
+  likely need to copy data into and out of it.  Any channel that allows
+  for more efficient processing on at least one end, e.g. through
+  zero-copy, is considered more efficient and thus preferred.  If the
+  back-end can provide such a channel, it should decide to use it.
+
+  The request payload contains parameters for the subsequent data
+  transfer, as described in the :ref:`Migrating back-end state
+  <migrating_backend_state>` section.
+
+  The value returned is both an indication for success, and whether a
+  file descriptor for a back-end-provided channel is returned: Bits 0–7
+  are 0 on success, and non-zero on error.  Bit 8 is the invalid FD
+  flag; this flag is set when there is no file descriptor returned.
+  When this flag is not set, the front-end must use the returned file
+  descriptor as its end of the transfer channel.  The back-end must not
+  both indicate an error and return a file descriptor.
+
+  Using this function requires prior negotiation of the
+  ``VHOST_USER_PROTOCOL_F_DEVICE_STATE`` feature.
+
+``VHOST_USER_CHECK_DEVICE_STATE``
+  :id: 43
+  :equivalent ioctl: N/A
+  :request payload: N/A
+  :reply payload: ``u64``
+
+  After transferring the back-end’s internal state during migration (see
+  the :ref:`Migrating back-end state <migrating_backend_state>`
+  section), check whether the back-end was able to successfully fully
+  process the state.
+
+  The value returned indicates success or error; 0 is success, any
+  non-zero value is an error.
+
+  Using this function requires prior negotiation of the
+  ``VHOST_USER_PROTOCOL_F_DEVICE_STATE`` feature.
+
 Back-end message types
 ----------------------
 
-- 
2.41.0

[Virtio-fs] [PATCH v5 5/7] vhost-user: Interface for migration state transfer

[Hanna Czenczek]

Add the interface for transferring the back-end's state during migration
as defined previously in vhost-user.rst.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 include/hw/virtio/vhost-backend.h |  24 +++++
 include/hw/virtio/vhost-user.h    |   1 +
 include/hw/virtio/vhost.h         |  78 ++++++++++++++++
 hw/virtio/vhost-user.c            | 146 ++++++++++++++++++++++++++++++
 hw/virtio/vhost.c                 |  37 ++++++++
 5 files changed, 286 insertions(+)

diff --git a/include/hw/virtio/vhost-backend.h b/include/hw/virtio/vhost-backend.h
index 1860b541d8..7d399ec418 100644
--- a/include/hw/virtio/vhost-backend.h
+++ b/include/hw/virtio/vhost-backend.h
@@ -26,6 +26,18 @@ typedef enum VhostSetConfigType {
     VHOST_SET_CONFIG_TYPE_MIGRATION = 1,
 } VhostSetConfigType;
 
+typedef enum VhostDeviceStateDirection {
+    /* Transfer state from back-end (device) to front-end */
+    VHOST_TRANSFER_STATE_DIRECTION_SAVE = 0,
+    /* Transfer state from front-end to back-end (device) */
+    VHOST_TRANSFER_STATE_DIRECTION_LOAD = 1,
+} VhostDeviceStateDirection;
+
+typedef enum VhostDeviceStatePhase {
+    /* The device (and all its vrings) is stopped */
+    VHOST_TRANSFER_STATE_PHASE_STOPPED = 0,
+} VhostDeviceStatePhase;
+
 struct vhost_inflight;
 struct vhost_dev;
 struct vhost_log;
@@ -133,6 +145,15 @@ typedef int (*vhost_set_config_call_op)(struct vhost_dev *dev,
 
 typedef void (*vhost_reset_status_op)(struct vhost_dev *dev);
 
+typedef bool (*vhost_supports_device_state_op)(struct vhost_dev *dev);
+typedef int (*vhost_set_device_state_fd_op)(struct vhost_dev *dev,
+                                            VhostDeviceStateDirection direction,
+                                            VhostDeviceStatePhase phase,
+                                            int fd,
+                                            int *reply_fd,
+                                            Error **errp);
+typedef int (*vhost_check_device_state_op)(struct vhost_dev *dev, Error **errp);
+
 typedef struct VhostOps {
     VhostBackendType backend_type;
     vhost_backend_init vhost_backend_init;
@@ -181,6 +202,9 @@ typedef struct VhostOps {
     vhost_force_iommu_op vhost_force_iommu;
     vhost_set_config_call_op vhost_set_config_call;
     vhost_reset_status_op vhost_reset_status;
+    vhost_supports_device_state_op vhost_supports_device_state;
+    vhost_set_device_state_fd_op vhost_set_device_state_fd;
+    vhost_check_device_state_op vhost_check_device_state;
 } VhostOps;
 
 int vhost_backend_update_device_iotlb(struct vhost_dev *dev,
diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
index 1d4121431b..324cd8663a 100644
--- a/include/hw/virtio/vhost-user.h
+++ b/include/hw/virtio/vhost-user.h
@@ -31,6 +31,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_STATUS = 16,
     /* Feature 17 reserved for VHOST_USER_PROTOCOL_F_XEN_MMAP. */
     VHOST_USER_PROTOCOL_F_SHARED_OBJECT = 18,
+    VHOST_USER_PROTOCOL_F_DEVICE_STATE = 19,
     VHOST_USER_PROTOCOL_F_MAX
 };
 
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 14621f9e79..a0d03c9fdf 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -348,4 +348,82 @@ static inline int vhost_reset_device(struct vhost_dev *hdev)
 }
 #endif /* CONFIG_VHOST */
 
+/**
+ * vhost_supports_device_state(): Checks whether the back-end supports
+ * transferring internal device state for the purpose of migration.
+ * Support for this feature is required for vhost_set_device_state_fd()
+ * and vhost_check_device_state().
+ *
+ * @dev: The vhost device
+ *
+ * Returns true if the device supports these commands, and false if it
+ * does not.
+ */
+bool vhost_supports_device_state(struct vhost_dev *dev);
+
+/**
+ * vhost_set_device_state_fd(): Begin transfer of internal state from/to
+ * the back-end for the purpose of migration.  Data is to be transferred
+ * over a pipe according to @direction and @phase.  The sending end must
+ * only write to the pipe, and the receiving end must only read from it.
+ * Once the sending end is done, it closes its FD.  The receiving end
+ * must take this as the end-of-transfer signal and close its FD, too.
+ *
+ * @fd is the back-end's end of the pipe: The write FD for SAVE, and the
+ * read FD for LOAD.  This function transfers ownership of @fd to the
+ * back-end, i.e. closes it in the front-end.
+ *
+ * The back-end may optionally reply with an FD of its own, if this
+ * improves efficiency on its end.  In this case, the returned FD is
+ * stored in *reply_fd.  The back-end will discard the FD sent to it,
+ * and the front-end must use *reply_fd for transferring state to/from
+ * the back-end.
+ *
+ * @dev: The vhost device
+ * @direction: The direction in which the state is to be transferred.
+ *             For outgoing migrations, this is SAVE, and data is read
+ *             from the back-end and stored by the front-end in the
+ *             migration stream.
+ *             For incoming migrations, this is LOAD, and data is read
+ *             by the front-end from the migration stream and sent to
+ *             the back-end to restore the saved state.
+ * @phase: Which migration phase we are in.  Currently, there is only
+ *         STOPPED (device and all vrings are stopped), in the future,
+ *         more phases such as PRE_COPY or POST_COPY may be added.
+ * @fd: Back-end's end of the pipe through which to transfer state; note
+ *      that ownership is transferred to the back-end, so this function
+ *      closes @fd in the front-end.
+ * @reply_fd: If the back-end wishes to use a different pipe for state
+ *            transfer, this will contain an FD for the front-end to
+ *            use.  Otherwise, -1 is stored here.
+ * @errp: Potential error description
+ *
+ * Returns 0 on success, and -errno on failure.
+ */
+int vhost_set_device_state_fd(struct vhost_dev *dev,
+                              VhostDeviceStateDirection direction,
+                              VhostDeviceStatePhase phase,
+                              int fd,
+                              int *reply_fd,
+                              Error **errp);
+
+/**
+ * vhost_set_device_state_fd(): After transferring state from/to the
+ * back-end via vhost_set_device_state_fd(), i.e. once the sending end
+ * has closed the pipe, inquire the back-end to report any potential
+ * errors that have occurred on its side.  This allows to sense errors
+ * like:
+ * - During outgoing migration, when the source side had already started
+ *   to produce its state, something went wrong and it failed to finish
+ * - During incoming migration, when the received state is somehow
+ *   invalid and cannot be processed by the back-end
+ *
+ * @dev: The vhost device
+ * @errp: Potential error description
+ *
+ * Returns 0 when the back-end reports successful state transfer and
+ * processing, and -errno when an error occurred somewhere.
+ */
+int vhost_check_device_state(struct vhost_dev *dev, Error **errp);
+
 #endif
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index dc2258c4ab..c6f86a4b27 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -103,6 +103,8 @@ typedef enum VhostUserRequest {
     VHOST_USER_SET_STATUS = 39,
     VHOST_USER_GET_STATUS = 40,
     VHOST_USER_GET_SHARED_OBJECT = 41,
+    VHOST_USER_SET_DEVICE_STATE_FD = 42,
+    VHOST_USER_CHECK_DEVICE_STATE = 43,
     VHOST_USER_MAX
 } VhostUserRequest;
 
@@ -201,6 +203,12 @@ typedef struct {
     uint32_t size; /* the following payload size */
 } QEMU_PACKED VhostUserHeader;
 
+/* Request payload of VHOST_USER_SET_DEVICE_STATE_FD */
+typedef struct VhostUserTransferDeviceState {
+    uint32_t direction;
+    uint32_t phase;
+} VhostUserTransferDeviceState;
+
 typedef union {
 #define VHOST_USER_VRING_IDX_MASK   (0xff)
 #define VHOST_USER_VRING_NOFD_MASK  (0x1 << 8)
@@ -216,6 +224,7 @@ typedef union {
         VhostUserVringArea area;
         VhostUserInflight inflight;
         VhostUserShared object;
+        VhostUserTransferDeviceState transfer_state;
 } VhostUserPayload;
 
 typedef struct VhostUserMsg {
@@ -2868,6 +2877,140 @@ static void vhost_user_reset_status(struct vhost_dev *dev)
     }
 }
 
+static bool vhost_user_supports_device_state(struct vhost_dev *dev)
+{
+    return virtio_has_feature(dev->protocol_features,
+                              VHOST_USER_PROTOCOL_F_DEVICE_STATE);
+}
+
+static int vhost_user_set_device_state_fd(struct vhost_dev *dev,
+                                          VhostDeviceStateDirection direction,
+                                          VhostDeviceStatePhase phase,
+                                          int fd,
+                                          int *reply_fd,
+                                          Error **errp)
+{
+    int ret;
+    struct vhost_user *vu = dev->opaque;
+    VhostUserMsg msg = {
+        .hdr = {
+            .request = VHOST_USER_SET_DEVICE_STATE_FD,
+            .flags = VHOST_USER_VERSION,
+            .size = sizeof(msg.payload.transfer_state),
+        },
+        .payload.transfer_state = {
+            .direction = direction,
+            .phase = phase,
+        },
+    };
+
+    *reply_fd = -1;
+
+    if (!vhost_user_supports_device_state(dev)) {
+        close(fd);
+        error_setg(errp, "Back-end does not support migration state transfer");
+        return -ENOTSUP;
+    }
+
+    ret = vhost_user_write(dev, &msg, &fd, 1);
+    close(fd);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret,
+                         "Failed to send SET_DEVICE_STATE_FD message");
+        return ret;
+    }
+
+    ret = vhost_user_read(dev, &msg);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret,
+                         "Failed to receive SET_DEVICE_STATE_FD reply");
+        return ret;
+    }
+
+    if (msg.hdr.request != VHOST_USER_SET_DEVICE_STATE_FD) {
+        error_setg(errp,
+                   "Received unexpected message type, expected %d, received %d",
+                   VHOST_USER_SET_DEVICE_STATE_FD, msg.hdr.request);
+        return -EPROTO;
+    }
+
+    if (msg.hdr.size != sizeof(msg.payload.u64)) {
+        error_setg(errp,
+                   "Received bad message size, expected %zu, received %" PRIu32,
+                   sizeof(msg.payload.u64), msg.hdr.size);
+        return -EPROTO;
+    }
+
+    if ((msg.payload.u64 & 0xff) != 0) {
+        error_setg(errp, "Back-end did not accept migration state transfer");
+        return -EIO;
+    }
+
+    if (!(msg.payload.u64 & VHOST_USER_VRING_NOFD_MASK)) {
+        *reply_fd = qemu_chr_fe_get_msgfd(vu->user->chr);
+        if (*reply_fd < 0) {
+            error_setg(errp,
+                       "Failed to get back-end-provided transfer pipe FD");
+            *reply_fd = -1;
+            return -EIO;
+        }
+    }
+
+    return 0;
+}
+
+static int vhost_user_check_device_state(struct vhost_dev *dev, Error **errp)
+{
+    int ret;
+    VhostUserMsg msg = {
+        .hdr = {
+            .request = VHOST_USER_CHECK_DEVICE_STATE,
+            .flags = VHOST_USER_VERSION,
+            .size = 0,
+        },
+    };
+
+    if (!vhost_user_supports_device_state(dev)) {
+        error_setg(errp, "Back-end does not support migration state transfer");
+        return -ENOTSUP;
+    }
+
+    ret = vhost_user_write(dev, &msg, NULL, 0);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret,
+                         "Failed to send CHECK_DEVICE_STATE message");
+        return ret;
+    }
+
+    ret = vhost_user_read(dev, &msg);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret,
+                         "Failed to receive CHECK_DEVICE_STATE reply");
+        return ret;
+    }
+
+    if (msg.hdr.request != VHOST_USER_CHECK_DEVICE_STATE) {
+        error_setg(errp,
+                   "Received unexpected message type, expected %d, received %d",
+                   VHOST_USER_CHECK_DEVICE_STATE, msg.hdr.request);
+        return -EPROTO;
+    }
+
+    if (msg.hdr.size != sizeof(msg.payload.u64)) {
+        error_setg(errp,
+                   "Received bad message size, expected %zu, received %" PRIu32,
+                   sizeof(msg.payload.u64), msg.hdr.size);
+        return -EPROTO;
+    }
+
+    if (msg.payload.u64 != 0) {
+        error_setg(errp, "Back-end failed to process its internal state");
+        return -EIO;
+    }
+
+    return 0;
+}
+
 const VhostOps user_ops = {
         .backend_type = VHOST_BACKEND_TYPE_USER,
         .vhost_backend_init = vhost_user_backend_init,
@@ -2904,4 +3047,7 @@ const VhostOps user_ops = {
         .vhost_set_inflight_fd = vhost_user_set_inflight_fd,
         .vhost_dev_start = vhost_user_dev_start,
         .vhost_reset_status = vhost_user_reset_status,
+        .vhost_supports_device_state = vhost_user_supports_device_state,
+        .vhost_set_device_state_fd = vhost_user_set_device_state_fd,
+        .vhost_check_device_state = vhost_user_check_device_state,
 };
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index bce97635df..ca4bdd9d66 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -2096,3 +2096,40 @@ int vhost_reset_device(struct vhost_dev *hdev)
 
     return -ENOSYS;
 }
+
+bool vhost_supports_device_state(struct vhost_dev *dev)
+{
+    if (dev->vhost_ops->vhost_supports_device_state) {
+        return dev->vhost_ops->vhost_supports_device_state(dev);
+    }
+
+    return false;
+}
+
+int vhost_set_device_state_fd(struct vhost_dev *dev,
+                              VhostDeviceStateDirection direction,
+                              VhostDeviceStatePhase phase,
+                              int fd,
+                              int *reply_fd,
+                              Error **errp)
+{
+    if (dev->vhost_ops->vhost_set_device_state_fd) {
+        return dev->vhost_ops->vhost_set_device_state_fd(dev, direction, phase,
+                                                         fd, reply_fd, errp);
+    }
+
+    error_setg(errp,
+               "vhost transport does not support migration state transfer");
+    return -ENOSYS;
+}
+
+int vhost_check_device_state(struct vhost_dev *dev, Error **errp)
+{
+    if (dev->vhost_ops->vhost_check_device_state) {
+        return dev->vhost_ops->vhost_check_device_state(dev, errp);
+    }
+
+    error_setg(errp,
+               "vhost transport does not support migration state transfer");
+    return -ENOSYS;
+}
-- 
2.41.0

[Virtio-fs] [PATCH v5 6/7] vhost: Add high-level state save/load functions

[Hanna Czenczek]

vhost_save_backend_state() and vhost_load_backend_state() can be used by
vhost front-ends to easily save and load the back-end's state to/from
the migration stream.

Because we do not know the full state size ahead of time,
vhost_save_backend_state() simply reads the data in 1 MB chunks, and
writes each chunk consecutively into the migration stream, prefixed by
its length.  EOF is indicated by a 0-length chunk.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 include/hw/virtio/vhost.h |  35 +++++++
 hw/virtio/vhost.c         | 204 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 239 insertions(+)

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index a0d03c9fdf..100fcc874d 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -426,4 +426,39 @@ int vhost_set_device_state_fd(struct vhost_dev *dev,
  */
 int vhost_check_device_state(struct vhost_dev *dev, Error **errp);
 
+/**
+ * vhost_save_backend_state(): High-level function to receive a vhost
+ * back-end's state, and save it in @f.  Uses
+ * `vhost_set_device_state_fd()` to get the data from the back-end, and
+ * stores it in consecutive chunks that are each prefixed by their
+ * respective length (be32).  The end is marked by a 0-length chunk.
+ *
+ * Must only be called while the device and all its vrings are stopped
+ * (`VHOST_TRANSFER_STATE_PHASE_STOPPED`).
+ *
+ * @dev: The vhost device from which to save the state
+ * @f: Migration stream in which to save the state
+ * @errp: Potential error message
+ *
+ * Returns 0 on success, and -errno otherwise.
+ */
+int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp);
+
+/**
+ * vhost_load_backend_state(): High-level function to load a vhost
+ * back-end's state from @f, and send it over to the back-end.  Reads
+ * the data from @f in the format used by `vhost_save_state()`, and uses
+ * `vhost_set_device_state_fd()` to transfer it to the back-end.
+ *
+ * Must only be called while the device and all its vrings are stopped
+ * (`VHOST_TRANSFER_STATE_PHASE_STOPPED`).
+ *
+ * @dev: The vhost device to which to send the sate
+ * @f: Migration stream from which to load the state
+ * @errp: Potential error message
+ *
+ * Returns 0 on success, and -errno otherwise.
+ */
+int vhost_load_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp);
+
 #endif
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index ca4bdd9d66..643c1af5a0 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -2133,3 +2133,207 @@ int vhost_check_device_state(struct vhost_dev *dev, Error **errp)
                "vhost transport does not support migration state transfer");
     return -ENOSYS;
 }
+
+int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
+{
+    /* Maximum chunk size in which to transfer the state */
+    const size_t chunk_size = 1 * 1024 * 1024;
+    g_autofree void *transfer_buf = NULL;
+    g_autoptr(GError) g_err = NULL;
+    int pipe_fds[2], read_fd = -1, write_fd = -1, reply_fd = -1;
+    int ret;
+
+    /* [0] for reading (our end), [1] for writing (back-end's end) */
+    if (!g_unix_open_pipe(pipe_fds, FD_CLOEXEC, &g_err)) {
+        error_setg(errp, "Failed to set up state transfer pipe: %s",
+                   g_err->message);
+        ret = -EINVAL;
+        goto fail;
+    }
+
+    read_fd = pipe_fds[0];
+    write_fd = pipe_fds[1];
+
+    /*
+     * VHOST_TRANSFER_STATE_PHASE_STOPPED means the device must be stopped.
+     * Ideally, it is suspended, but SUSPEND/RESUME currently do not exist for
+     * vhost-user, so just check that it is stopped at all.
+     */
+    assert(!dev->started);
+
+    /* Transfer ownership of write_fd to the back-end */
+    ret = vhost_set_device_state_fd(dev,
+                                    VHOST_TRANSFER_STATE_DIRECTION_SAVE,
+                                    VHOST_TRANSFER_STATE_PHASE_STOPPED,
+                                    write_fd,
+                                    &reply_fd,
+                                    errp);
+    if (ret < 0) {
+        error_prepend(errp, "Failed to initiate state transfer: ");
+        goto fail;
+    }
+
+    /* If the back-end wishes to use a different pipe, switch over */
+    if (reply_fd >= 0) {
+        close(read_fd);
+        read_fd = reply_fd;
+    }
+
+    transfer_buf = g_malloc(chunk_size);
+
+    while (true) {
+        ssize_t read_ret;
+
+        read_ret = RETRY_ON_EINTR(read(read_fd, transfer_buf, chunk_size));
+        if (read_ret < 0) {
+            ret = -errno;
+            error_setg_errno(errp, -ret, "Failed to receive state");
+            goto fail;
+        }
+
+        assert(read_ret <= chunk_size);
+        qemu_put_be32(f, read_ret);
+
+        if (read_ret == 0) {
+            /* EOF */
+            break;
+        }
+
+        qemu_put_buffer(f, transfer_buf, read_ret);
+    }
+
+    /*
+     * Back-end will not really care, but be clean and close our end of the pipe
+     * before inquiring the back-end about whether transfer was successful
+     */
+    close(read_fd);
+    read_fd = -1;
+
+    /* Also, verify that the device is still stopped */
+    assert(!dev->started);
+
+    ret = vhost_check_device_state(dev, errp);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = 0;
+fail:
+    if (read_fd >= 0) {
+        close(read_fd);
+    }
+
+    return ret;
+}
+
+int vhost_load_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp)
+{
+    size_t transfer_buf_size = 0;
+    g_autofree void *transfer_buf = NULL;
+    g_autoptr(GError) g_err = NULL;
+    int pipe_fds[2], read_fd = -1, write_fd = -1, reply_fd = -1;
+    int ret;
+
+    /* [0] for reading (back-end's end), [1] for writing (our end) */
+    if (!g_unix_open_pipe(pipe_fds, FD_CLOEXEC, &g_err)) {
+        error_setg(errp, "Failed to set up state transfer pipe: %s",
+                   g_err->message);
+        ret = -EINVAL;
+        goto fail;
+    }
+
+    read_fd = pipe_fds[0];
+    write_fd = pipe_fds[1];
+
+    /*
+     * VHOST_TRANSFER_STATE_PHASE_STOPPED means the device must be stopped.
+     * Ideally, it is suspended, but SUSPEND/RESUME currently do not exist for
+     * vhost-user, so just check that it is stopped at all.
+     */
+    assert(!dev->started);
+
+    /* Transfer ownership of read_fd to the back-end */
+    ret = vhost_set_device_state_fd(dev,
+                                    VHOST_TRANSFER_STATE_DIRECTION_LOAD,
+                                    VHOST_TRANSFER_STATE_PHASE_STOPPED,
+                                    read_fd,
+                                    &reply_fd,
+                                    errp);
+    if (ret < 0) {
+        error_prepend(errp, "Failed to initiate state transfer: ");
+        goto fail;
+    }
+
+    /* If the back-end wishes to use a different pipe, switch over */
+    if (reply_fd >= 0) {
+        close(write_fd);
+        write_fd = reply_fd;
+    }
+
+    while (true) {
+        size_t this_chunk_size = qemu_get_be32(f);
+        ssize_t write_ret;
+        const uint8_t *transfer_pointer;
+
+        if (this_chunk_size == 0) {
+            /* End of state */
+            break;
+        }
+
+        if (transfer_buf_size < this_chunk_size) {
+            transfer_buf = g_realloc(transfer_buf, this_chunk_size);
+            transfer_buf_size = this_chunk_size;
+        }
+
+        if (qemu_get_buffer(f, transfer_buf, this_chunk_size) <
+                this_chunk_size)
+        {
+            error_setg(errp, "Failed to read state");
+            ret = -EINVAL;
+            goto fail;
+        }
+
+        transfer_pointer = transfer_buf;
+        while (this_chunk_size > 0) {
+            write_ret = RETRY_ON_EINTR(
+                write(write_fd, transfer_pointer, this_chunk_size)
+            );
+            if (write_ret < 0) {
+                ret = -errno;
+                error_setg_errno(errp, -ret, "Failed to send state");
+                goto fail;
+            } else if (write_ret == 0) {
+                error_setg(errp, "Failed to send state: Connection is closed");
+                ret = -ECONNRESET;
+                goto fail;
+            }
+
+            assert(write_ret <= this_chunk_size);
+            this_chunk_size -= write_ret;
+            transfer_pointer += write_ret;
+        }
+    }
+
+    /*
+     * Close our end, thus ending transfer, before inquiring the back-end about
+     * whether transfer was successful
+     */
+    close(write_fd);
+    write_fd = -1;
+
+    /* Also, verify that the device is still stopped */
+    assert(!dev->started);
+
+    ret = vhost_check_device_state(dev, errp);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    ret = 0;
+fail:
+    if (write_fd >= 0) {
+        close(write_fd);
+    }
+
+    return ret;
+}
-- 
2.41.0

[Virtio-fs] [PATCH v5 7/7] vhost-user-fs: Implement internal migration

[Hanna Czenczek]

A virtio-fs device's VM state consists of:
- the virtio device (vring) state (VMSTATE_VIRTIO_DEVICE)
- the back-end's (virtiofsd's) internal state

We get/set the latter via the new vhost operations to transfer migratory
state.  It is its own dedicated subsection, so that for external
migration, it can be disabled.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
---
 hw/virtio/vhost-user-fs.c | 101 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 100 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
index 49d699ffc2..eb91723855 100644
--- a/hw/virtio/vhost-user-fs.c
+++ b/hw/virtio/vhost-user-fs.c
@@ -298,9 +298,108 @@ static struct vhost_dev *vuf_get_vhost(VirtIODevice *vdev)
     return &fs->vhost_dev;
 }
 
+/**
+ * Fetch the internal state from virtiofsd and save it to `f`.
+ */
+static int vuf_save_state(QEMUFile *f, void *pv, size_t size,
+                          const VMStateField *field, JSONWriter *vmdesc)
+{
+    VirtIODevice *vdev = pv;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+    Error *local_error = NULL;
+    int ret;
+
+    ret = vhost_save_backend_state(&fs->vhost_dev, f, &local_error);
+    if (ret < 0) {
+        error_reportf_err(local_error,
+                          "Error saving back-end state of %s device %s "
+                          "(tag: \"%s\"): ",
+                          vdev->name, vdev->parent_obj.canonical_path,
+                          fs->conf.tag ?: "<none>");
+        return ret;
+    }
+
+    return 0;
+}
+
+/**
+ * Load virtiofsd's internal state from `f` and send it over to virtiofsd.
+ */
+static int vuf_load_state(QEMUFile *f, void *pv, size_t size,
+                          const VMStateField *field)
+{
+    VirtIODevice *vdev = pv;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+    Error *local_error = NULL;
+    int ret;
+
+    ret = vhost_load_backend_state(&fs->vhost_dev, f, &local_error);
+    if (ret < 0) {
+        error_reportf_err(local_error,
+                          "Error loading back-end state of %s device %s "
+                          "(tag: \"%s\"): ",
+                          vdev->name, vdev->parent_obj.canonical_path,
+                          fs->conf.tag ?: "<none>");
+        return ret;
+    }
+
+    return 0;
+}
+
+static bool vuf_is_internal_migration(void *opaque)
+{
+    /* TODO: Return false when an external migration is requested */
+    return true;
+}
+
+static int vuf_check_migration_support(void *opaque)
+{
+    VirtIODevice *vdev = opaque;
+    VHostUserFS *fs = VHOST_USER_FS(vdev);
+
+    if (!vhost_supports_device_state(&fs->vhost_dev)) {
+        error_report("Back-end of %s device %s (tag: \"%s\") does not support "
+                     "migration through qemu",
+                     vdev->name, vdev->parent_obj.canonical_path,
+                     fs->conf.tag ?: "<none>");
+        return -ENOTSUP;
+    }
+
+    return 0;
+}
+
+static const VMStateDescription vuf_backend_vmstate;
+
 static const VMStateDescription vuf_vmstate = {
     .name = "vhost-user-fs",
-    .unmigratable = 1,
+    .version_id = 0,
+    .fields = (VMStateField[]) {
+        VMSTATE_VIRTIO_DEVICE,
+        VMSTATE_END_OF_LIST()
+    },
+    .subsections = (const VMStateDescription * []) {
+        &vuf_backend_vmstate,
+        NULL,
+    }
+};
+
+static const VMStateDescription vuf_backend_vmstate = {
+    .name = "vhost-user-fs-backend",
+    .version_id = 0,
+    .needed = vuf_is_internal_migration,
+    .pre_load = vuf_check_migration_support,
+    .pre_save = vuf_check_migration_support,
+    .fields = (VMStateField[]) {
+        {
+            .name = "back-end",
+            .info = &(const VMStateInfo) {
+                .name = "virtio-fs back-end state",
+                .get = vuf_load_state,
+                .put = vuf_save_state,
+            },
+        },
+        VMSTATE_END_OF_LIST()
+    },
 };
 
 static Property vuf_properties[] = {
-- 
2.41.0