Re: [virtio-comment] Re: [virtio] [PATCH] virtio: clarify feature negotiation

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jan 24 2022, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> It's true. However, that's a generic rule and specific devices
> can differ. and if you look at specific instances, you will
> see that they typically require that the feature is negotiated.
> For example:
>
> \item If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated,
>   \field{blk_size} can be read to determine the optimal sector size
>   for the driver to use.
>
> so a driver reading blk_size before FEATURES_OK is actually using
> an undocumented aspect of the behaviour.
>
> compare this to MTU:
>
> \item[VIRTIO_NET_F_MTU(3)] Device maximum MTU reporting is supported. If
>     offered by the device, device advises driver about the value of
>     its maximum MTU. If negotiated, the driver uses \field{mtu} as
>     the maximum MTU value.
>
> from which it's clear that it can be read even if not
> negotiated.
>
> So from that point of view, we are actually relaxing the requirements,
> and yes we'll need to carefully go over each instance of
> "offered".
> I thought I did, but now I did a quick grep again and I found instances
> in virtio-iommu.tex and virtio-gpio.tex
> Both of these are new, so I think we can just fix them to "negotiated".

Considering this, should we actually add any requirements as to when
fields are valid? The device may present a value that is always valid if
it offers the corresponding feature, or that value may change depending
on what is actually negotiated by the driver. Maybe a statement that the
actual validity of a field's value before negotiation is device type
dependant?