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

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.
>> +
>> +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.
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?

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