Currently QEMU has to know some details about the back-end to be able
to setup the guest. While various parts of the setup can be delegated
to the backend (for example config handling) this is a very piecemeal
approach.
This patch suggests a new feature flag (VHOST_USER_PROTOCOL_F_STANDALONE)
which the back-end can advertise which allows a probe message to be
sent to get all the details QEMU needs to know in one message.
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
Initial RFC for discussion. I intend to prototype this work with QEMU
and one of the rust-vmm vhost-user daemons.
---
docs/interop/vhost-user.rst | 37 +++++++++++++++++++++++++++++++++++++
hw/virtio/vhost-user.c | 8 ++++++++
2 files changed, 45 insertions(+)
diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 5a070adbc1..85b1b1583a 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -275,6 +275,21 @@ Inflight description
:queue size: a 16-bit size of virtqueues
+Backend specifications
+^^^^^^^^^^^^^^^^^^^^^^
+
++-----------+-------------+------------+------------+
+| device id | config size | min_vqs | max_vqs |
++-----------+-------------+------------+------------+
+
+:device id: a 32-bit value holding the VirtIO device ID
+
+:config size: a 32-bit value holding the config size (see
``VHOST_USER_GET_CONFIG``)
+
+:min_vqs: a 32-bit value holding the minimum number of vqs supported
+
+:max_vqs: a 32-bit value holding the maximum number of vqs supported, must be
>= min_vqs
+
C structure
-----------
@@ -296,6 +311,7 @@ In QEMU the vhost-user message is implemented with the
following struct:
VhostUserConfig config;
VhostUserVringArea area;
VhostUserInflight inflight;
+ VhostUserBackendSpecs specs;
};
} QEMU_PACKED VhostUserMsg;
@@ -316,6 +332,7 @@ replies. Here is a list of the ones that do:
* ``VHOST_USER_GET_VRING_BASE``
* ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
* ``VHOST_USER_GET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
+* ``VHOST_USER_GET_BACKEND_SPECS`` (if ``VHOST_USER_PROTOCOL_F_STANDALONE``)
.. seealso::
@@ -885,6 +902,13 @@ Protocol features
#define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS 15
#define VHOST_USER_PROTOCOL_F_STATUS 16
#define VHOST_USER_PROTOCOL_F_XEN_MMAP 17
+ #define VHOST_USER_PROTOCOL_F_STANDALONE 18
+
+Some features are only valid in the presence of other supporting
+features. In the case of ``VHOST_USER_PROTOCOL_F_STANDALONE`` the
+backend must also support ``VHOST_USER_PROTOCOL_F_CONFIG`` and
+``VHOST_USER_PROTOCOL_F_STATUS``.
+