On Wed, Aug 05, 2020 at 03:13:06PM +0000, Alyssa Ross wrote: > Quoting from the definition of VHOST_USER_SET_PROTOCOL_FEATURES in > vhost-user.rst: > > > Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in > > ``VHOST_USER_GET_FEATURES``. > > > > .. Note:: > > Slave that reported ``VHOST_USER_F_PROTOCOL_FEATURES`` must support > > this message even before ``VHOST_USER_SET_FEATURES`` was called. > > To me, this could mean either of two things: > > (1) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > VHOST_USER_SET_PROTOCOL_FEATURES, a backend should enable the > protocol features immediately. > > (2) If VHOST_USER_F_PROTOCOL_FEATURES hasn't been set, upon receiving > VHOST_USER_SET_PROTOCOL_FEATURES, a backend should store those > feature bits, but not actually consider them to be enabled until > after VHOST_USER_SET_FEATURES has been received (presumably > containing VHOST_USER_F_PROTOCOL_FEATURES). > > The reason I bring this up is that QEMU appears to interpret it as (1), > while the vhost-user-net backend in Intel's cloud-hypervisor[1] > interprets it as (2). So I'm looking for a clarification. > > [1]: https://github.com/cloud-hypervisor/cloud-hypervisor > > Thanks in advance.
IMHO the intent was this: VHOST_USER_F_PROTOCOL_FEATURES bit in VHOST_USER_GET_FEATURES means that qemu can send VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. With most feature bits in VHOST_USER_GET_FEATURES, the specific functionality needs to only be enabled after VHOST_USER_SET_FEATURES. However, this is for functionality dealing with guest activity. VHOST_USER_SET_PROTOCOL_FEATURES has nothing to do with guest directly, it's about negotiation between qemu and backend: it is only in VHOST_USER_GET_FEATURES for the reason that this is the only message (very) old backends reported. Thus, the backend should not check whether VHOST_USER_SET_FEATURES sets VHOST_USER_F_PROTOCOL_FEATURES, instead it should simply always be ready to receive VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES. Backend that isn't always ready to handle VHOST_USER_GET_PROTOCOL_FEATURES and VHOST_USER_SET_PROTOCOL_FEATURES should not set VHOST_USER_F_PROTOCOL_FEATURES in VHOST_USER_GET_FEATURES. This appears to be closer to (1), but if qemu can't distinguish then we don't care, right? For example, VHOST_USER_PROTOCOL_F_REPLY_ACK enables acks on arbitrary messages. Does the backend in question ignore the affected bit until SET_FEATURES? If yes won't this make qemu hang? How would you suggest clarifying the wording? Thanks, -- MST
