[virtio-comment] Re: [PATCH v1 3/8] device-context: Define the device context fields for device migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Oct 08, 2023 at 02:25:50PM +0300, Parav Pandit wrote:
> +\begin{lstlisting}
> +struct virtio_dev_ctx_pci_vq_cfg {
> +        le16 vq_index;
> +        le16 queue_size;
> +        le16 queue_msix_vector;
> +        le64 queue_desc;
> +        le64 queue_driver;
> +        le64 queue_device;
> +};
> +\end{lstlisting}
> +
> +One or multiple entries of PCI Virtqueue Configuration Context may exist, each such
> +entry corresponds to a unique virtqueue identified by the \field{vq_index}.

So consider this example. In practice it is quite possible that
driver is in the process of specifying e.g. queue_desc, and it
set queue_desc_hi but not queue_desc_lo. Then what is queue_desc?
Just a combination of a legal value of queue_desc_hi and
illegal one of queue_desc_lo? This makes no sense.
queue_desc is fundamentally undefined until queue is enabled.

This is why I suggest that we have records that match
the transport. Offset in structure can then we used as a tag and
so we do not need to come up with new definitions for each single thing.

And, this is only an instance of the general principle: do not
have two definitions of the same thing. In fact I'd argue our
transport structures are an example of a bad design and the
cost is that less used ones like mmio and ccw sometimes lag behind
on features.


-- 
MST


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