Re: [PATCH 1/2] virtio: introduce selective queue enabling

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

On Tue, Jun 06, 2023 at 07:55:10PM +0200, Eugenio Pérez wrote:
> This patch allows the driver to start the device (as set DRIVER_OK) with only
> some queues enabled, and then enable another queues later.
> 
> This is the current way to migrate net device state through control
> virtqueue, in a software assisted framework with vDPA:
> * First, only net CVQ is enabled at DRIVER_OK
> * All the control commands (mac address, mq, etc) needed for the device
> to behave the same as the source of migration are sent
> * Finally all the dataplane queues are enabled.
> 
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>  content.tex | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index d2ab9eb..ea4a6d8 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -99,10 +99,10 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B
>  \begin{description}
>  \item[0 to 23, and 50 to 127] Feature bits for the specific device type
>  
> -\item[24 to 41] Feature bits reserved for extensions to the queue and
> +\item[24 to 42] Feature bits reserved for extensions to the queue and
>    feature negotiation mechanisms
>  
> -\item[42 to 49, and 128 and above] Feature bits reserved for future extensions.
> +\item[43 to 49, and 128 and above] Feature bits reserved for future extensions.
>  \end{description}
>  
>  \begin{note}
> @@ -398,6 +398,13 @@ \section{Virtqueues}\label{sec:Basic Facilities of a Virtio Device / Virtqueues}
>  Every driver and device supports either the Packed or the Split
>  Virtqueue format, or both.
>  
> +\subsection{Virtqueue selective enabling}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Selective Virtqueue Enable}
> +
> +If the driver negotiates VIRTIO\_F\_RING\_ENABLE\_ANYTIME,

No need for \_, we build with _ not being a special character.

> it can enable
> +individual virtqueues both before and after the DRIVER\_OK. The way to enable a
> +virtqueue is transport specific, but it mimics the re-enabling after an
> +hypothetical reset.
> +


This is not a good way to do it: spec describes a certain order
of initialization in several places. Please find all of them
and fix, don't just add a separate chapter as a kind of kludge.


>  \subsection{Virtqueue Reset}\label{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}
>  
>  When VIRTIO_F_RING_RESET is negotiated, the driver can reset a virtqueue
> @@ -870,6 +877,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>  	\ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits} for
>  	handling features reserved for future use.
>  
> +  \item[VIRTIO_F_RING_ENABLE_ANYTIME(42)] This feature indicates
> +  that the driver can enable a queue both before and after DRIVER\_OK.
> +  See \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Selective Virtqueue Enable}.
> +
>  \end{description}
>  
>  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> -- 
> 2.31.1