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Subject: Re: [virtio-dev] [VHOST USER SPEC PATCH] vhost-user.rst: add clarifying language about protocol negotiation
On Mon, Mar 01, 2021 at 11:38:47AM +0000, Alex Bennée wrote: > Stefan Hajnoczi <stefanha@redhat.com> writes: > > On Fri, Feb 26, 2021 at 11:16:19AM +0000, Alex Bennée wrote: > >> +However as the protocol negotiation something that only occurs between > > > > Missing "is". Shortening the sentence fixes that without losing clarity: > > s/something that/negotiation/ > > > >> +parts of the backend implementation it is permissible to for the master > > > > "vhost-user device backend" is often used to refer to the slave (to > > avoid saying the word "slave") but "backend" is being used in a > > different sense here. That is confusing. > > > >> +to mask the feature bit from the guest. > > > > I think this sentence effectively says "the master MAY mask the > > VHOST_USER_F_PROTOCOL_FEATURES bit from the VIRTIO feature bits". That > > is not really accurate since VIRTIO devices do not advertise this > > feature bit and so it can never be negotiated through the VIRTIO feature > > negotiation process. > > > > How about referring to the details from the VIRTIO 1.1 specification > > instead. Something like this: > > > > Note that VHOST_USER_F_PROTOCOL_FEATURES is the UNUSED (30) feature > > bit defined in `VIRTIO 1.1 6.3 Legacy Interface: Reserved Feature Bits > > <https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-4130003>`_. > > VIRTIO devices do not advertise this feature bit and therefore VIRTIO > > drivers cannot negotiate it. > > > > This reserved feature bit was reused by the vhost-user protocol to add > > vhost-user protocol feature negotiation in a backwards compatible > > fashion. Old vhost-user master and slave implementations continue to > > work even though they are not aware of vhost-user protocol feature > > negotiation. > > OK - so does that mean that feature bit will remain UNUSED for ever > more? It's unlikely to be repurposed in VIRTIO. It can never be used by VIRTIO in a situation that overlaps with vhost-user. That leaves cases that don't overlap with vhost-user but that is unlikely too since the bit had a previous meaning (before vhost-user) and repurposing it would cause confusion for very old drivers or devices. > What about other feature bits? Is it permissible for the > master/requester/vhost-user front-end/QEMU to filter any other feature > bits the slave/vhost-user backend/daemon may offer from being read by > the guest driver when it reads the feature bits? Yes, the vhost-user frontend can decide how it wants to expose VHOST_USER_GET_FEATURES feature bits on the VIRTIO device: 1. Pass-through. Allow the vhost-user device backend to control the feature bit. 2. Disabling. Clear a feature bit because it cannot be supported for some reason (e.g. VIRTIO 1.1 packed vrings are not implemented and therefore enabling them would prevent live migration). 3. Enabling. Enable a feature bit that does not rely on vhost-user device backend support. For example, message-signalled interrupts for virtio-mmio. > > > > >> As noted for the > >> +``VHOST_USER_GET_PROTOCOL_FEATURES`` and > >> +``VHOST_USER_SET_PROTOCOL_FEATURES`` messages this occurs before a > >> +final ``VHOST_USER_SET_FEATURES`` comes from the guest. > > > > I couldn't find any place where vhost-user.rst states that > > VHOST_USER_SET_PROTOCOL_FEATURES has to come before > > VHOST_USER_SET_FEATURES? > > > > The only order I found was: > > > > 1. VHOST_USER_GET_FEATURES to determine whether protocol features are > > supported. > > 2. VHOST_USER_GET_PROTOCOL_FEATURES to fetch available protocol feature bits. > > 3. VHOST_USER_SET_PROTOCOL_FEATURES to set protocol feature bits. > > 4. Using functionality that depends on enabled protocol feature bits. > > > > Is the purpose of this sentence to add a new requirement to the spec > > that "VHOST_USER_SET_PROTOCOL_FEATURES MUST be sent before > > VHOST_USER_SET_FEATURES"? > > No I don't want to add a new sequence requirement. But if SET_FEATURES > doesn't acknowledge the VHOST_USER_F_PROTOCOL_FEATURES bit should that > stop the processing of > VHOST_USER_GET_PROTOCOL_FEATURES/VHOST_USER_SET_PROTOCOL_FEATURES > messages? AFAICT SET_FEATURES should be irrelevant to the negotiation of > the PROTOCOL_FEATURES right? I agree, the value of VHOST_USER_F_PROTOCOL_FEATURES in VHOST_USER_SET_FEATURES does not matter according to the spec: Only legal if feature bit ``VHOST_USER_F_PROTOCOL_FEATURES`` is present in ``VHOST_USER_GET_FEATURES``. Since it does not mention "set in VHOST_USER_SET_FEATURES" we have to assume existing vhost-user device backends do not care whether the vhost-user frontend includes the bit in VHOST_USER_SET_FEATURES or not. Stefan
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