diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/migration.rst | 15 | ||||
| -rw-r--r-- | docs/interop/qmp-spec.txt | 42 |
2 files changed, 37 insertions, 20 deletions
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index 6ed3fce061..687570754d 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -240,10 +240,13 @@ should succeed even with the data missing. To support this the subsection can be connected to a device property and from there to a versioned machine type. -One important note is that the post_load() function is called "after" -loading all subsections, because a newer subsection could change same -value that it uses. A flag, and the combination of pre_load and post_load -can be used to detect whether a subsection was loaded, and to +The 'pre_load' and 'post_load' functions on subsections are only +called if the subsection is loaded. + +One important note is that the outer post_load() function is called "after" +loading all subsections, because a newer subsection could change the same +value that it uses. A flag, and the combination of outer pre_load and +post_load can be used to detect whether a subsection was loaded, and to fall back on default behaviour when the subsection isn't present. Example: @@ -315,8 +318,8 @@ For example: the property to false. c) Add a static bool support_foo function that tests the property. d) Add a subsection with a .needed set to the support_foo function - e) (potentially) Add a pre_load that sets up a default value for 'foo' - to be used if the subsection isn't loaded. + e) (potentially) Add an outer pre_load that sets up a default value + for 'foo' to be used if the subsection isn't loaded. Now that subsection will not be generated when using an older machine type and the migration stream will be accepted by older diff --git a/docs/interop/qmp-spec.txt b/docs/interop/qmp-spec.txt index 1566b8ae5e..8f7da0245d 100644 --- a/docs/interop/qmp-spec.txt +++ b/docs/interop/qmp-spec.txt @@ -20,9 +20,9 @@ operating system. 2. Protocol Specification ========================= -This section details the protocol format. For the purpose of this document -"Client" is any application which is using QMP to communicate with QEMU and -"Server" is QEMU itself. +This section details the protocol format. For the purpose of this +document, "Server" is either QEMU or the QEMU Guest Agent, and +"Client" is any application communicating with it via QMP. JSON data structures, when mentioned in this document, are always in the following format: @@ -34,9 +34,8 @@ by the JSON standard: http://www.ietf.org/rfc/rfc7159.txt -The protocol is always encoded in UTF-8 except for synchronization -bytes (documented below); although thanks to json-string escape -sequences, the server will reply using only the strict ASCII subset. +The server expects its input to be encoded in UTF-8, and sends its +output encoded in ASCII. For convenience, json-object members mentioned in this document will be in a certain order. However, in real protocol usage they can be in @@ -215,16 +214,31 @@ Some events are rate-limited to at most one per second. If additional dropped, and the last one is delayed. "Similar" normally means same event type. See qmp-events.txt for details. -2.6 QGA Synchronization +2.6 Forcing the JSON parser into known-good state +------------------------------------------------- + +Incomplete or invalid input can leave the server's JSON parser in a +state where it can't parse additional commands. To get it back into +known-good state, the client should provoke a lexical error. + +The cleanest way to do that is sending an ASCII control character +other than '\t' (horizontal tab), '\r' (carriage return), or '\n' (new +line). + +Sadly, older versions of QEMU can fail to flag this as an error. If a +client needs to deal with them, it should send a 0xFF byte. + +2.7 QGA Synchronization ----------------------- -When using QGA, an additional synchronization feature is built into -the protocol. If the Client sends a raw 0xFF sentinel byte (not valid -JSON), then the Server will reset its state and discard all pending -data prior to the sentinel. Conversely, if the Client makes use of -the 'guest-sync-delimited' command, the Server will send a raw 0xFF -sentinel byte prior to its response, to aid the Client in discarding -any data prior to the sentinel. +When a client connects to QGA over a transport lacking proper +connection semantics such as virtio-serial, QGA may have read partial +input from a previous client. The client needs to force QGA's parser +into known-good state using the previous section's technique. +Moreover, the client may receive output a previous client didn't read. +To help with skipping that output, QGA provides the +'guest-sync-delimited' command. Refer to its documentation for +details. 3. QMP Examples |