RE: [PATCH linux-next v3 2/6] vdpa: Introduce query of device config layout

[Parav Pandit <parav@nvidia.com>]

> From: Jason Wang <jasowang@redhat.com>
> Sent: Wednesday, June 23, 2021 9:39 AM
> 
> 在 2021/6/22 下午10:03, Parav Pandit 写道:
> >> Is it better to use a separate enum for net specific attributes?
> >>
> > Yes, because they are only net specific.
> > I guess it is related to your below question.
> >
> >> Another question (sorry if it has been asked before). Can we simply
> >> return the config (binary) to the userspace, then usespace can use
> >> the existing uAPI like virtio_net_config plus the feature to explain the
> config?
> >>
> > We did discuss in v2.
> > Usually returning the whole blob and parsing is not desired via netlink.
> > Returning individual fields give the full flexibility to return only the valid
> fields.
> > Otherwise we need to implement another bitmask too to tell which fields
> from the struct are valid and share with user space.
> > Returning individual fields is the widely used approach.
> 
> 
> The main concerns are:
> 
> 1) The blob will be self contained if it was passed with the negotiated
> features, so we don't need bitmask.
Which fields of the struct are valid is told by additional fields.
> 2) Using individual fields means it must duplicate the config fields of every
> virtio devices
> 
Mostly no. if there are common config fields across two device types, they would be named as
VDPA_ATTR_DEV_CFG_*
Net specific will be,
VDPA_ATTR_DEV_NET_CFG_*
Block specific, will be,
VDPA_ATTR_DEV_BLK_CFG_*

> And actually, it's not the binary blob since uapi clearly define the format (e.g
> struct virtio_net_config), can we find a way to use that? E.g introduce
> device/net specific command and passing the blob with length and
> negotiated features.
Length may change in future, mostly expand. And parsing based on length is not such a clean way.
Parsing fields require knowledge of features as well and application needs to make multiple netlink calls to parse the config space.
I prefer to follow rest of the kernel style to return self contained invidividual fields.
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