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

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

On 8/14/2023 10:49 PM, Stefan Hajnoczi wrote:
> 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"
Yes, will fix
> > +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."
we did not define the used_state for non-packed vqs in this series.
I will define this in the next version. and Say it is always 0 if 
VIRTIO_F_PACKED is not negotiated.
> > +
> > +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"
Yes, will fix

Thanks
> > +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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