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

On Thu, 26 Mar 2020 11:32:14 +0100 Bjoern Walk <bwalk(a)linux.ibm.com&gt; 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>