[virtio-comment] [PATCH V2 1/6] virtio: introduce virtqueue state

[Zhu Lingshan <lingshan.zhu@intel.com>]

This patch adds new virtqueue facility to save and restore virtqueue
state. The virtqueue state is split into two parts:

- The available state: The state that is used for read the next
  available buffer.
- The used state: The state that is used for make buffer used.

This will simply the transport specific method implementation. E.g two
le16 could be used instead of a single le32). For split virtqueue, we
only need the available state since the used state is implemented in
the virtqueue itself (the used index). For packed virtqueue, we need
both the available state and the used state.

The typical use cases are live migration and debugging.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 content.tex     |  7 ++++--
 packed-ring.tex | 58 +++++++++++++++++++++++++++++++++++++++++++++++++
 split-ring.tex  | 39 +++++++++++++++++++++++++++++++++
 3 files changed, 102 insertions(+), 2 deletions(-)

diff --git a/content.tex b/content.tex
index 0a62dce..76813b5 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}
@@ -872,6 +872,9 @@ \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_QUEUE_STATE(42)] This feature indicates that the device allows the driver
+  to access its internal virtqueue state.
+
 \end{description}
 
 \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
diff --git a/packed-ring.tex b/packed-ring.tex
index 9eeb382..ad6aba0 100644
--- a/packed-ring.tex
+++ b/packed-ring.tex
@@ -729,3 +729,61 @@ \subsection{Receiving Used Buffers From The Device}\label{sec:Basic Facilities o
         process_buffer(d);
 }
 \end{lstlisting}
+
+\subsection{Virtqueue State}\label{sec:Basic Facilities of a Virtio Device / Packed Virtqueues / Virtqueue State}
+
+When VIRTIO_F_QUEUE_STATE has been negotiated, the driver can set and
+get the device internal virtqueue state through the following
+fields. The implementation of the interfaces is transport specific.
+
+\subsubsection{\field{Available State} Field}
+
+The available state field is two bytes of virtqueue state that is used by
+the device to read the next available buffer. It is presented in the followwing format:
+
+\begin{lstlisting}
+le16 {
+  last_avail_idx : 15;
+  last_avail_wrap_counter : 1;
+};
+\end{lstlisting}
+
+The \field{last_avail_idx} field is the free-running location
+where the device read the next descriptor from the virtqueue descriptor ring.
+
+The \field{last_avail_wrap_counter} field is the last driver ring wrap
+counter that was observed by the device.
+
+See also \ref{sec:Packed Virtqueues / Driver and Device Ring Wrap Counters}.
+
+\subsubsection{\field{Used State} Field}
+
+The used state field is two bytes of virtqueue state that is used by
+the device when marking a buffer used. It is presented in the followwing format:
+
+\begin{lstlisting}
+le16 {
+  used_idx : 15;
+  used_wrap_counter : 1;
+};
+\end{lstlisting}
+
+The \field{used_idx} field is the free-running location where the device write the next
+used descriptor to the descriptor ring.
+
+The \field{used_wrap_counter} field is the wrap counter that is used
+by the device.
+
+See also \ref{sec:Packed Virtqueues / Driver and Device Ring Wrap Counters}.
+
+\devicenormative{\subsubsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Packed Virtqueues/ Virtqueue State}
+
+The device SHOULD only accept setting Virtqueue State of any packed virtqueues when DRIVER_OK is not set in \field{device status}, or SUSPEND is set in \field{device status}.
+Otherwise the device MUST ignore any writes to Virtqueue State of any packed virtqueues.
+
+When SUSPEND is set, the device MUST record the Virtqueue State of every enabled packed virtqueue
+in \field{Available State} field and \field{Used State} field respectively,
+and correspondingly restore the Virtqueue State of every enabled packed virtqueue
+from \field{Available State} field and \field{Used State} field when DRIVER_OK is set.
+
+The device SHOULD reset \field{Available State} field and \field{Used State} field upon a device reset.
diff --git a/split-ring.tex b/split-ring.tex
index de94038..a78b44d 100644
--- a/split-ring.tex
+++ b/split-ring.tex
@@ -734,3 +734,42 @@ \subsection{Receiving Used Buffers From The Device}\label{sec:Basic Facilities o
 }
 \end{lstlisting}
 \end{note}
+
+\subsection{Virtqueue State}\label{sec:Basic Facilities of a Virtio Device / Splited Virtqueues / Virtqueue State}
+
+When VIRTIO_F_QUEUE_STATE has been negotiated, the driver can set and
+get the device internal virtqueue state through the following
+fields. The implementation of the interfaces is transport specific.
+
+\subsubsection{\field{Available State} Field}
+
+The available state field is two bytes of virtqueue state that is used by
+the device to read the next available buffer. It is presented in the followwing format:
+
+\begin{lstlisting}
+le16 last_avail_idx;
+\end{lstlisting}
+
+The \field{last_avail_idx} field is the free-running available ring
+index where the device will read the next available head of a
+descriptor chain.
+
+See also \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Available Ring}.
+
+\drivernormative{\subsubsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Splited Virtqueues/ Virtqueue State}
+
+The driver SHOULD NOT access \field{Used State} of any splited virtqueues, it SHOULD use the
+used index in the used ring.
+
+\devicenormative{\subsubsection}{Virtqueue State}{Basic Facilities of a Virtio Device / Splited Virtqueues/ Virtqueue State}
+
+The device SHOULD only accept setting Virtqueue State of any splited virtqueues
+when DRIVER_OK is not set in \field{device status} or SUSPEND is set in \field{device status}.
+Otherwise the device MUST ignore any writes to Virtqueue State of any splited virtqueues.
+
+When SUSPEND is set, the device MUST record the Available State of every enabled splited virtqueue
+in \field{Available State} field,
+and correspondingly restore the Available State of every enabled splited virtqueue
+from \field{Available State} field when DRIVER_OK is set.
+
+The device SHOULD reset \field{Available State} field upon a device reset.
-- 
2.35.3


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