On Thu, Aug 06, 2020 at 08:59:09AM +0000, Alyssa Ross wrote: > "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
Hmm I find this confusing. I think it's a good policy to ask qemu to acknowledge it. It's just that the client should not wait for VHOST_USER_SET_FEATURES before handling VHOST_USER_SET_PROTOCOL_FEATURES or VHOST_USER_GET_PROTOCOL_FEATURES. > @@ -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
