[virtio-comment] 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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