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

On Mon, Nov 14, 2022 at 04:46:54PM -0600, Eric Blake wrote: > #### 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. Given that we are introducing this at the same time as the extended headers (and given that we're running out of available flags in this 16-bit field), isn't it better to make the ability to understand PAYLOAD_LEN be implied by extended headers? Or is there a use case for implementing extended headers but not a payload length on CMD_BLOCK_STATUS that I'm missing?