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Subject: Re: [virtio-comment] [PATCH 1/5] virtio: introduce vq state as basic facility
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). 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. virtqueue state can not, generally, be described by two 16 bit indices. Consider an example: these buffers available: A B C D After device used descriptors A and C, what is its state and how do you describe it using a single index? > + > +\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. next after what? > + > +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. I don't get what good does this used_idx do - used descriptors are written in order so just check which ones are valid? And driver does of course know what the used_wrap_counter is otherwise it can't work. Or is this for some kind of split driver setup where looking at the ring is impossible? > + > +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 > > > This publicly archived list offers a means to provide input to the > OASIS Virtual I/O Device (VIRTIO) TC. > > In order to verify user consent to the Feedback License terms and > to minimize spam in the list archive, subscription is required > before posting. > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > List help: virtio-comment-help@lists.oasis-open.org > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > Committee: https://www.oasis-open.org/committees/virtio/ > Join OASIS: https://www.oasis-open.org/join/ >
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