Re: [virtio-dev] [RFC PATCH 2/5] virtio: introduce vq state as basic facility

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Aug 15, 2023 at 03:29:01AM +0800, Zhu Lingshan wrote:
> This patch adds new device 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 implemention. 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.
> 
> Those states are required to implement live migration support for
> virtio device.
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> ---
>  content.tex | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 62 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index 1bb4401..074f43e 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -516,6 +516,68 @@ \section{Exporting Objects}\label{sec:Basic Facilities of a Virtio Device / Expo
>  types. It is RECOMMENDED that devices generate version 4
>  UUIDs as specified by \hyperref[intro:rfc4122]{[RFC4122]}.
>  
> +\section{Virtqueue State}\label{sec:Virtqueues / Virtqueue State}
> +
> +When VIRTIO_F_QUEUE_STATE is negotiated, the driver can set and
> +get the device internal virtqueue state through the following
> +fields. The implementation of the interfaces is transport specific.
> +
> +\subsection{\field{Available State} Field}
> +
> +The available state field is two bytes virtqueue state that is used by

"two bytes of virtqueue state"

> +the device to read the next available buffer.
> +
> +When VIRTIO_RING_F_PACKED is not negotiated, it contains:
> +
> +\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}.
> +
> +When VIRTIO_RING_F_PACKED is negotiated, it contains:
> +
> +\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}.
> +
> +\subsection{\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.

Please specify the non-packed case:

"When VIRTIO_RING_F_PACKED is not negotiated, the 16-bit value is always
0."

> +
> +When VIRTIO_RING_F_PACKED is negotiated, the used state contains:
> +
> +\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 next

"device writes 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}.
> +
>  \input{admin.tex}
>  
>  \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
> -- 
> 2.35.3
> 
> 
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