[virtio-comment] Re: [PATCH 2/5] virtio: introduce SUSPEND bit in device status

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 06, 2023 at 04:16:34PM +0800, Zhu Lingshan wrote:
> This patch introduces a new status bit in the device status: SUSPEND.
> 
> This SUSPEND bit can be used by the driver to suspend a device,
> in order to stabilize the device states and virtqueue states.
> 
> Its main use case is live migration.
> 
> 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 | 31 +++++++++++++++++++++++++++++++
>  1 file changed, 31 insertions(+)
> 
> diff --git a/content.tex b/content.tex
> index 0e492cd..0fab537 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -47,6 +47,9 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
>  \item[DRIVER_OK (4)] Indicates that the driver is set up and ready to
>    drive the device.
>  
> +\item[SUSPEND (16)] When VIRTIO_F_SUSPEND is negotiated, indicates that the
> +  device has been suspended by the driver.
> +
>  \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has experienced
>    an error from which it can't recover.
>  \end{description}
> @@ -73,6 +76,10 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
>  recover by issuing a reset.
>  \end{note}
>  
> +The driver SHOULD NOT set SUSPEND if FEATURES_OK is not set.
> +
> +When setting SUSPEND, the driver MUST re-read \field{device status} to ensure the SUSPEND bit is set.
> +
>  \devicenormative{\subsection}{Device Status Field}{Basic Facilities of a Virtio Device / Device Status Field}
>  
>  The device MUST NOT consume buffers or send any used buffer
> @@ -82,6 +89,26 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
>  that a reset is needed.  If DRIVER_OK is set, after it sets DEVICE_NEEDS_RESET, the device
>  MUST send a device configuration change notification to the driver.
>  
> +The device MUST ignore SUSPEND if FEATURES_OK is not set.
> +
> +The device MUST ignore SUSPEND if VIRTIO_F_SUSPEND is not negotiated.

why? let's just forbid driver from setting it.

> +
> +The device SHOULD allow settings to \field{device status} even when SUSPEND is set.
> +
> +If VIRTIO_F_SUSPEND is negotiated and SUSPEND is set, the device SHOULD clear SUSPEND
> +and resumes operation upon DRIVER_OK.
> +

sorry what?

> +If VIRTIO_F_SUSPEND is negotiated, when the driver sets SUSPEND,
> +the device SHOULD perform the following actions before presenting SUSPEND bit in the \field{device status}:
> +
> +\begin{itemize}
> +\item Stop consuming buffers of any virtqueues and mark all finished descritors as used.
> +\item Wait until all descriptors that being processed to finish and mark them as used.
> +\item Flush all used buffer and send used buffer notifications to the driver.

flush how?

> +\item Record Virtqueue State of each enabled virtqueue, see section \ref{sec:Virtqueues / Virtqueue State}


record where?

> +\item Pause its operation except \field{device status} and preserve configurations in its Device Configuration Space, see \ref{sec:Basic Facilities of a Virtio Device / Device Configuration Space}

pause in what sense? completely?  this does not seem realistic.
e.g. pci express link has to stay active or device will die.


also, presumably here it is except a bunch of other fields.
e.g. what about queue select and all related queue fields?


> +\end{itemize}
> +
>  \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature Bits}
>  
>  Each virtio device offers all the features it understands.  During
> @@ -937,6 +964,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>  	\ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits} for
>  	handling features reserved for future use.
>  
> +  \item[VIRTIO_F_SUSPEND(42)] This feature indicates that the driver can
> +   SUSPEND the device.
> +   See \ref{sec:Basic Facilities of a Virtio Device / Device Status Field}.
> +
>  \end{description}
>  
>  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> -- 
> 2.35.3


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