Re: [virtio-dev] Re: [PATCH v2] content: Introduce VIRTIO_NET_F_STANDBY feature

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 25 Jul 2018 16:31:05 -0700
Siwei Liu <loseweigh@gmail.com> wrote:

> On Tue, Jul 24, 2018 at 5:43 AM, Cornelia Huck <cohuck@redhat.com> wrote:
> > On Mon, 23 Jul 2018 10:02:35 -0700
> > Sridhar Samudrala <sridhar.samudrala@intel.com> wrote:
> >  
> >> VIRTIO_NET_F_STANDBY feature enables hypervisor to indicate virtio_net
> >> device to act as a standby for a primary device with the same MAC address.
> >>
> >> Signed-off-by: Sridhar Samudrala <sridhar.samudrala@intel.com
> >> ---
> >>  content.tex | 8 ++++++++
> >>  1 file changed, 8 insertions(+)
> >>
> >> v2: updated standby description based on Cornelia's feedback.
> >>
> >> diff --git a/content.tex b/content.tex
> >> index be18234..b729857 100644
> >> --- a/content.tex
> >> +++ b/content.tex
> >> @@ -2525,6 +2525,9 @@ features.
> >>
> >>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
> >>      channel.
> >> +
> >> +\item[VIRTIO_NET_F_STANDBY(62)] Device MAY act as a standby for a primary
> >> +    device with the same MAC  
> >
> > I don't think you should use MAY etc. outside a normative section, so
> > s/MAY/may/
> >  
> >>  \end{description}
> >>
> >>  \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements}
> >> @@ -2636,6 +2639,11 @@ If the driver negotiates VIRTIO_NET_F_MTU, it MUST NOT transmit packets of
> >>  size exceeding the value of \field{mtu} (plus low level ethernet header length)
> >>  with \field{gso_type} NONE or ECN.
> >>
> >> +A driver SHOULD negotiate VIRTIO_NET_F_STANDBY feature if the device offers it.  
> >
> > s/VIRTIO_NET_F_STANDBY feature/the VIRTIO_NET_F_STANDBY feature/
> >  
> >> +
> >> +If the driver negotiates VIRTIO_NET_F_STANDBY, the device MAY act as a standby
> >> +device for a primary device with the same MAC address.  
> >
> > I think the first statement needs to go into a driver normative
> > section, while the second needs to go into a device normative section.
> >  
> >> +
> >>  \subsubsection{Legacy Interface: Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout / Legacy Interface: Device configuration layout}
> >>  \label{sec:Device Types / Block Device / Feature bits / Device configuration layout / Legacy Interface: Device configuration layout}
> >>  When using the legacy interface, transitional devices and drivers  
> >
> > I still think we need a more detailed description of how this is
> > supposed to work elsewhere (i.e., outside of the normative section).
> > But we can probably merge an updated version of this patch to get at
> > least the feature bit reserved and documented. Thoughts?  
> 
> I don't understand the purpose of this spec. Nothing has been
> discussed and described beyond the current guest implementation.

That's exactly the purpose of this update: Define what is actually the
de facto meaning of the new feature bit.

> Formerly I would expect to see more descriptions on the device side
> behaviour: how primary device may or may not get exposed depending on
> feature negotiation result of the standby device, how the primary
> device may behave or get exposed if the driver for standby virtio
> initiates a device reset, et al.  But from the last discussion I got
> the impression that the host-guest interface is frozen whenever the
> guest implementation is shipped and that behaviour becomes the spec.
> Currently the guest implementation in Linux 4.18 is to NOT interact
> with device side for anything during feature negotiaion. I don't see
> what needs to be merged for an updated version. The current
> VIRTIO_NET_F_STANDBY is irrelevant to anything in the device side, so
> what needs to be reserved?

It is *not* irrelevant to the device side: If a device offers the
feature bit, it might trigger behaviour in the guest (i.e. looking for
a device with a matching MAC address). As soon as you have a feature
bit, the device and the driver *are* interacting.

This is just specifying the minimum needed to make sure that
implementations in the host are not making existing code out of spec.