On 04/14/2016 01:25 AM, Wouter Verhelst wrote: > Hi Eric, > > On Wed, Apr 13, 2016 at 06:37:29PM -0600, Eric Blake wrote: >> Since NBD_OPT_INFO and NBD_OPT_GO are experimental, we still have >> a chance to fix them up before promoting them to stable. >> >> Attempting to reuse NBD_OPT_SERVER as the reply to NBD_OPT_INFO and >> NBD_OPT_GO has a few problems: clients must be prepared to parse >> two different styles of the reply, based on which option request >> the reply is answering. Extending the information to provide even >> more details, like block sizing, is awkward (the only way to do it >> within a single reply is to have multiple length fields that must >> all be consistent; and pre-computing the overall header length may >> be difficult). And requiring the server to parrot back the export >> name is wasteful if the client's name is already in canonical >> form. > > Right. > >> Solve this by instead making the valid response be a series of reply >> messages (similar to how NBD_OPT_LIST has a series). The series >> is always ended by the new NBD_REP_EXPORT, which is basically a > > If we're going to make it a series of messages, I would prefer that the > final message be an NBD_REP_ACK. That way, wire sniffers can always spot > the end of a multi-message reply by way of the ACK at the end, even if > they don't understand the actual multi-message reply.
Doable, but then we lose the nice property that the tail 10 bytes of reply to NBD_OPT_GO match the tail 10 bytes of NBD_OPT_EXPORT_NAME. But that property isn't a show-stopper, so I can go ahead and make that change (basically, just add NBD_REP_INFO and not NBD_REP_EXPORT; declare that NBD_INFO_EXPORT is a mandatory reply, and that other infos are optional). -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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