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

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

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On 8/16/2023 10:05 AM, Jason Wang wrote:
> On Tue, Aug 15, 2023 at 6:50 PM Zhu, Lingshan<lingshan.zhu@intel.com>  wrote:
>>
>>
>> On 8/15/2023 8:37 AM, Jason Wang wrote:
>>> On Tue, Aug 15, 2023 at 8:26 AM Jason Wang<jasowang@redhat.com>  wrote:
>>>> On Mon, Aug 14, 2023 at 7:29 PM Zhu Lingshan<lingshan.zhu@intel.com>  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.
>>>> I think patch 3 needs to be squashed into this one.
>>>>
>>>>> Signed-off-by: Jason Wang<jasowang@redhat.com>
>>>>> Signed-off-by: Eugenio PÃrez<eperezma@redhat.com>
>>>>> Signed-off-by: Zhu Lingshan<lingshan.zhu@intel.com>
>>>>> ---
>>>>>    content.tex | 18 ++++++++++++++++++
>>>>>    1 file changed, 18 insertions(+)
>>>>>
>>>>> 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.
>>>>> +
>>>>>    \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.
>>>> Is there any value to allow SUSPEND to be set without DRIVER_OK?
>>>>
>>>>> +
>>>>> +The deivce MUST ignore SUSPEND if VIRTIO_F_SUSPEND is not negotiated.
>>>> typo.
>>>>
>>>> Thanks
>>> Btw, it's not clear to me if driver is allowed to clear this bit.
>> To allow terminate a live migration process or resume from a failed live
>> migration,
>> we allow DRIVER_OK to clear SUSPEND, in patch 1.
> Does this imply DRIVER_OK is cleared during SUSPEND? This seems more
> complicated than simply allowing clearing this bit?
No, SUSPEND does not clears DRIVER_OK, only DRIVER_OK clears SUSPEND.
The spec says: The driver MUST NOT clear a device status bit.
So I think maybe it's better let the device clears SUSPEND.

Thanks
>
> Thanks
>
>> Thanks
>>> Thanks
>>>
>>>>> +
>>>>> +If VIRTIO_F_SUSPEND is negotiated and SUSPEND is set, the device MUST clear SUSPEND
>>>>> +and resumes operation upon DRIVER_OK.
>>>>> +
>>>>>    \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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