"Michael S. Tsirkin" <m...@redhat.com> writes: > 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.
Thanks for the explanation. That matches what I had in mind with (1). > 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? Yes. That was my motivation for asking what the correct behaviour was, so that I could fix the incorrect one. :) I suspect that up to this point, the cloud-hypervisor vhost-user-net backend has only been used with cloud-hypervisor, and so this incompatibilty with QEMU was not noticed. > How would you suggest clarifying the wording? Do you think this communicates everything required? --- diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst index 10e3e3475e..72724d292a 100644 --- i/docs/interop/vhost-user.rst +++ w/docs/interop/vhost-user.rst @@ -854,9 +854,8 @@ Master message types ``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. + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged + with ``VHOST_USER_SET_FEATURES``. ``VHOST_USER_SET_PROTOCOL_FEATURES`` :id: 16 @@ -869,8 +868,8 @@ Master message types ``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. + ``VHOST_USER_F_PROTOCOL_FEATURES`` does not need to be acknowledged + with ``VHOST_USER_SET_FEATURES``. ``VHOST_USER_SET_OWNER`` :id: 3
