[virtio-dev] Re: [virtio-comment] [PATCH 1/5] virtio: introduce vq state as basic facility

Michael S. Tsirkin
2023-09-14T11:25:37Z

On Wed, Sep 06, 2023 at 04:16:33PM +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 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).

hmm no, simply because when ring is not running and when
buffers are processed in order, last avail == used.

> 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: Zhu Lingshan <lingshan.zhu@intel.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>

> ---
> content.tex | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 65 insertions(+)
>
> diff --git a/content.tex b/content.tex
> index 0a62dce..0e492cd 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -502,6 +502,71 @@ \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 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.
> +
> +\subsection{\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.
> +
> +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.

I dislike how this pokes at split-ring.txt externally.
Will make it harder to add new formats down the road.

> +
> +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.
> +
> +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 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}.
> +
> +When VIRTIO_RING_F_PACKED is not negotiated, the 16-bit value of \field{used_idx}
> +is always 0
> +
> \input{admin.tex}
>
> \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation}
> --
> 2.35.3
>
>
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