On Thu, Aug 06, 2020 at 11:24:59AM +0000, Alyssa Ross wrote: > "Michael S. Tsirkin" <m...@redhat.com> writes: > > > 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. > > To me, it's confusing that a frontend is expected to ack > VHOST_USER_F_PROTOCOL_FEATURES even though the ack can't have any effect > (because VHOST_USER_GET_PROTOCOL_FEATURES and > VHOST_USER_SET_PROTOCOL_FEATURES both have to work even if the ack > hasn't been received yet). > > But, if the frontend is supposed to ack anyway, how about: > > Signed-off-by: Alyssa Ross <h...@alysas.is> > > --- > diff --git i/docs/interop/vhost-user.rst w/docs/interop/vhost-user.rst > index 10e3e3475e..bc78c9947f 100644 > --- i/docs/interop/vhost-user.rst > +++ w/docs/interop/vhost-user.rst > @@ -854,9 +854,9 @@ 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. > + While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a > + backend must allow ``VHOST_USER_GET_PROTOCOL_FEATURES`` even if > + ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet. > > ``VHOST_USER_SET_PROTOCOL_FEATURES`` > :id: 16 > @@ -869,8 +869,12 @@ 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. > + While QEMU should acknowledge ``VHOST_USER_F_PROTOCOL_FEATURES``, a > + backend must allow ``VHOST_USER_SET_PROTOCOL_FEATURES`` even if > + ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been acknowledged yet. > + The backend must not wait for ``VHOST_USER_SET_FEATURES`` before > + enabling protocol features requested with > + ``VHOST_USER_SET_PROTOCOL_FEATURES``. > > ``VHOST_USER_SET_OWNER`` > :id: 3
That looks good to me. Pls post a patch on list, preferably after qemu is released.
