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| -rw-r--r-- | docs/interop/qmp-spec.txt | 42 |
1 files changed, 28 insertions, 14 deletions
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 |