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

[Eugenio PÃrez <eperezma@redhat.com>]

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 PÃrez <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, 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.
+
 \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