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

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

On 8/14/2023 10:30 PM, Stefan Hajnoczi wrote:
> 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.
Oh, I copied his SOB form his email, I will copy from git log to fix this,
thanks for point out it.
>
>> 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?
It depends on the driver, normally we expect this operation can be done 
successfully
like how the driver/device handles FEATURES_OK.

Once failed due to:
1) driver timeout, the driver can reset the device
2) device failure, the device can set NEEDS_RESET.
>
> Does the driver need to re-read the Device Status Field after clearing
> the SUSPEND bit?
I think the driver should re-read, I will add this in the next version.
>
>> 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, ...".
Will fix in the next version.
>
>> +
>>   \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?
I expect DRIVER_OK can clear SUSPEND, so that the device can resume running
in case of a failed live migration.

Maybe I should say: DRIVER_OK clears SUSPEND, and if DRIVER_OK is set to
a suspended device, the device should resume operation

Thanks
>
>> +
>>   \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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