"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
