Re: [Libguestfs] [PATCH] spec: Add NBD_OPT_EXTENDED_HEADERS

On Mon, Dec 06, 2021 at 02:40:45PM +0300, Vladimir Sementsov-Ogievskiy wrote: > > #### Simple reply message > > > > The simple reply message MUST be sent by the server in response to all > > requests if structured replies have not been negotiated using > > -`NBD_OPT_STRUCTURED_REPLY`. If structured replies have been negotiated, a simple > > -reply MAY be used as a reply to any request other than `NBD_CMD_READ`, > > -but only if the reply has no data payload. The message looks as > > -follows: > > +`NBD_OPT_STRUCTURED_REPLY`. If structured replies have been > > +negotiated, a simple reply MAY be used as a reply to any request other > > +than `NBD_CMD_READ`, but only if the reply has no data payload. If > > +extended headers were not negotiated using `NBD_OPT_EXTENDED_HEADERS`, > > +the message looks as follows: > > > > S: 32 bits, 0x67446698, magic (`NBD_SIMPLE_REPLY_MAGIC`; used to be > > `NBD_REPLY_MAGIC`) > > @@ -369,6 +398,16 @@ S: 64 bits, handle > > S: (*length* bytes of data if the request is of type `NBD_CMD_READ` and > > *error* is zero) > > > > +If extended headers were negotiated using `NBD_OPT_EXTENDED_HEADERS`, > > +the message looks like: > > + > > +S: 32 bits, 0x60d12fd6, magic (`NBD_SIMPLE_REPLY_EXT_MAGIC`) > > +S: 32 bits, error (MAY be zero) > > +S: 64 bits, handle > > +S: 128 bits, padding (MUST be zero) > > +S: (*length* bytes of data if the request is of type `NBD_CMD_READ` and > > + *error* is zero) > > + > > If we go this way, let's put payload length into padding: it will help to make the protocol context-independent and less error-prone.

Easy enough to do (the payload length will be 0 except for NBD_CMD_READ).

> Or, the otherway, may be just forbid the payload for simple-64bit ? What's the reason to allow 64bit requests without structured reply negotiation? The two happened to be orthogonal enough in my implementation. It was easy to demonstrate either one without the other, and it IS easier to write a client using non-structured replies (structured reads ARE tougher than simple reads, even if it is less efficient when it comes to reading zeros). But you are also right that we could require structured reads prior to allowing 64-bit operations, and then have only one supported reply type on the wire when negotiated. Wouter, which way do you prefer?