[PATCH 4/4] docs: documentation for virtio packed option

[Halil Pasic]

On Thu, 26 Mar 2020 11:32:14 +0100
Bjoern Walk <bwalk at linux.ibm.com> wrote:

> +    <p>
> +      The attribute <code>packed</code> controls the usage of packed
> +      virtqueues. 

This reads to me, like if packed='on' is specified, then packed
virtqueues are used (or the device fails); if  packed='off' is specified
is specified then split virtqueue format is used.

But that statement is not always true. One might still end up using
split despite of packed='on'. One example is a setup vhost-net setup.
The in kernel implementation namely does not support packed queues, so
F_RING_PACKED does not get offered by the device.

Another reason why one could end up with F_RING_PACKED not getting
negotiated is lack of guest support.

Please note that this behavior is a little different than what we do for
cpus (and cpu models). There one either gets what he asked for, or the
VM won't get constructed.

I believe, we should at least document this cleanly. Cc-ing Michael
and Jason as hypervisor experts on these topics.

Regards,
Halil



> Compared to regular split queues, packed queues consist of
> +      only a single descriptor ring replacing available and used ring, index
> +      and descriptor buffer. This can result in better cache utilization and
> +      performance. Possible values are <code>on</code> or <code>off</code>.
> +      <span class="since">Since 6.3.0 (QEMU and KVM only)</span>
> +    </p>