Re: [virtio-comment] [RFC PATCH 1/5] virtio: introduce SUSPEND bit in device status

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Aug 15, 2023 at 03:29:00AM +0800, Zhu Lingshan wrote:
> This patch introudces 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 stablize the device states and virtqueue states.
> 
> Its main use case is live migration.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Eugenio PÃrez <eperezma@redhat.com>

There is an character encoding issue in Eugenio's surname.

> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
> ---
>  content.tex | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)

This patch hints at the asynchronous nature of the SUSPEND bit (the
driver must re-read the Device Status Field) but doesn't explain the
rationale or any limits.

For example, is there a timeout or should the driver re-read the Device
Status Field forever?

Does the driver need to re-read the Device Status Field after clearing
the SUSPEND bit?

> 
> diff --git a/content.tex b/content.tex
> index 0a62dce..1bb4401 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 MUST NOT set SUSPEND if FEATURES_OK is not set.
> +
> +When set SUSPEND, the driver MUST re-read \field{device status} to ensure the SUSPEND bit is set.

"When setting SUSPEND, ..." would be grammatically correct. Another
option is "After setting the SUSPEND bit, ...".

> +
>  \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 +89,13 @@ \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 deivce MUST ignore SUSPEND if VIRTIO_F_SUSPEND is not negotiated.
> +
> +If VIRTIO_F_SUSPEND is negotiated and SUSPEND is set, the device MUST clear SUSPEND
> +and resumes operation upon DRIVER_OK.

I can't parse this sentence. If the driver writes SUSPEND | DRIVER_OK |
... to the Device Status Field, then the device accepts DRIVER_OK and
clears SUSPEND?

Why?

> +
>  \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature Bits}
>  
>  Each virtio device offers all the features it understands.  During
> @@ -872,6 +886,10 @@ \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(41)] 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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