Re: [Libguestfs] [PATCH v2 5/6] spec: Introduce NBD_FLAG_BLOCK_STATUS_PAYLOAD

#### Simple reply message @@ -1232,6 +1235,19 @@ The field has the following format: will be faster than a regular write). Clients MUST NOT set the `NBD_CMD_FLAG_FAST_ZERO` request flag unless this transmission flag is set. +- bit 12, `NBD_FLAG_BLOCK_STATUS_PAYLOAD`: Indicates that the server + understands the use of the `NBD_CMD_FLAG_PAYLOAD_LEN` flag to + `NBD_CMD_BLOCK_STATUS` to allow the client to request that the + server filters its response to a specific subset of negotiated + metacontext ids passed in via a client payload, rather than the + default of replying to all metacontext ids. Servers MUST NOT + advertise this bit unless the client successfully negotiates + extended headers via `NBD_OPT_EXTENDED_HEADERS`, and SHOULD NOT + advertise this bit in response to `NBD_OPT_EXPORT_NAME` or + `NBD_OPT_GO` if the client does not negotiate metacontexts with + `NBD_OPT_SET_META_CONTEXT`; clients SHOULD NOT set the + `NBD_CMD_FLAG_PAYLOAD_LEN` flag for `NBD_CMD_BLOCK_STATUS` unless + this transmission flag is set.

Clients SHOULD ignore unknown flags. @@ -1915,8 +1931,11 @@ valid may depend on negotiation during the handshake phase. header. With extended headers, the flag MUST be set for `NBD_CMD_WRITE` (as the write command always sends a payload of the bytes to be written); for other commands, the flag will trigger an - `NBD_EINVAL` error unless the server has advertised support for an - extension payload form for the command. + `NBD_EINVAL` error unless the command documents an optional payload + form for the command and the server has implemented that form (an + example being `NBD_CMD_BLOCK_STATUS` providing a payload form for + restricting the response to a particular metacontext id, when the + server advertises `NBD_FLAG_BLOCK_STATUS_PAYLOAD`). ##### Structured reply flags @@ -2464,6 +2483,23 @@ The following request types exist: The server MAY send chunks in a different order than the context ids were assigned in reply to `NBD_OPT_SET_META_CONTEXT`. + If extended headers were negotiated, a server MAY optionally + advertise, via the transmission flag + `NBD_FLAG_BLOCK_STATUS_PAYLOAD`, that it supports an alternative + request form where the client sets `NBD_CMD_FLAG_PAYLOAD_LEN` in + order to pass a payload that informs the server to limit its + replies to the metacontext id(s) in the client's request payload, + rather than giving an answer on all possible metacontext ids. If + the server does not support the payload form, or detects duplicate + or unknown metacontext ids in the client's payload, the server + MUST gracefully consume the client's payload before failing with + `NBD_EINVAL`. The payload form MUST occupy 8 + n*4 bytes, where n + is the number of metacontext ids the client is interested in (as + implied by the payload length), laid out as: + + 64 bits, effect length + n * 32 bits, list of metacontext ids to use + The list of block status descriptors within a given status chunk represent consecutive portions of the file starting from specified *offset*. If the client used the `NBD_CMD_FLAG_REQ_ONE` flag,