[virtio-dev] Re: [PATCH] virtio: introduce SUSPEND bit in device status

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

On 2/20/2024 3:48 PM, Michael S. Tsirkin wrote:
> On Tue, Feb 20, 2024 at 02:09:18PM +0900, David Stevens wrote:
>> On Tue, Feb 20, 2024 at 1:06 PM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:
>>> On 2/19/2024 2:46 PM, David Stevens wrote:
>>>> On Sun, Feb 18, 2024 at 11:11 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>> On Sun, Feb 18, 2024 at 09:23:06PM +0800, Zhu Lingshan wrote:
>>>>>> This commit allows the driver to suspend the device by
>>>>>> introducing a new status bit SUSPEND in device_status.
>>>>>>
>>>>>> This commit also introduce a new feature bit VIRTIO_F_SUSPEND
>>>>>> which indicating whether the device support SUSPEND.
>>>>>>
>>>>>> This SUSPEND bit is transport-independent.
>>>>>>
>>>>>> 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>
>>>>> Could we get some kind of dscription how this has taken into
>>>>> consideration the proposal from David Stevens?
>>>>>
>>>>> I find it really tiring when there are competing patches with authors
>>>>> ignoring each other's work and leaving it up to reviewers to
>>>>> figure out how do the patches compare.
>>>> This patch looks like it could be used to implement my use case.
>>>> However, parts of it are a bit vague and imprecise, so it's hard to
>>>> actually say whether my use case would actually be covered by a
>>>> specific implementation of this proposal.
>>> I am on vacation till this Friday. Shall we co-work on this and
>>> post something new together?
>> That works for me.
>>
>>>> To give specifics on what I'm trying to do, I need to allow a guest
>>>> with virtio-pci devices (including stateful devices like virtio-fs and
>>>> virtio-gpu) to be put into S1/S3, and to allow the virtio-pci devices
>>>> to wake up the guest (currently via an ACPI GPE, but eventually via a
>>>> native PCI PME). This is all done on a consumer device, so there is no
>>>> need for snapshotting or for live migration.
>>> Suspending is not dedicated only for live migration.
>>> For your use case, shall we add a new PCI section like saying: when entering
>>> SUSPEND, the device should get into S1/S3 and other PCI specific steps
>>> in PCI transport charter.
>>>
>>> That is because PCI is a transport layer of virtio, controlling virtio
>>> states by PCI sounds like a layer violation.
>> I don't know if it's really necessary to mandate that in the spec.
>> Sure, most systems are probably going to want to put the virtio-pci
>> device into S1/S3 after suspending virtio operation. But nothing
>> really breaks or goes wrong if we don't do that.
> What I feel is worth mentioning in virtio spec is that just setting the
> status bit does not reduce power by itself, standard pci
> control is required for that.
I agree, only mention power management in a separate PCI transport patch in
a new collaborated series
>
>>>>>> ---
>>>>>>    content.tex | 34 ++++++++++++++++++++++++++++++++--
>>>>>>    1 file changed, 32 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/content.tex b/content.tex
>>>>>> index 0a62dce..3d656b5 100644
>>>>>> --- a/content.tex
>>>>>> +++ b/content.tex
>>>>>> @@ -49,6 +49,9 @@ \section{\field{Device Status} Field}\label{sec:Basic Facilities of a Virtio Dev
>>>>>>
>>>>>>    \item[DEVICE_NEEDS_RESET (64)] Indicates that the device has experienced
>>>>>>      an error from which it can't recover.
>>>>>> +
>>>>>> +\item[SUSPEND (16)] When VIRTIO_F_SUSPEND is negotiated, indicates that the
>>>>>> +  device has been suspended by the driver.
>>>>>>    \end{description}
>>>>>>
>>>>>>    The \field{device status} field starts out as 0, and is reinitialized to 0 by
>>>>>> @@ -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 +89,25 @@ \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.
>>>>>> +
>>>>>> +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.
>>>>>> +
>>>>>> +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 descriptors 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.
>>>> What's the difference between the previous three lines? They might be
>>>> trying to say different things, but there is a lot of overlap in how
>>>> they're phrased. That makes it hard to figure out exactly what they're
>>>> mandating. Is it something like: "stop processing new buffers",
>>>> "finish processing any buffers that are currently in flight", "mark
>>>> all finished buffers as used and send a used buffer notification"?
>>> sure, the "mark used" parts can merge.
>>>> Also, what does this mean for devices where the driver places empty
>>>> buffers into the virtqueue that the device then holds on to for an
>>>> indeterminate period of time (e.g. network receiveq, balloon statsq,
>>>> etc)?
>>> The device doesn't process them.
>>> Oh, I should add: The driver should not make any more buffers available
>>> to the device.
>>>>>> +\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}
>>>> What does "Pause its operation except device status" mean? The word
>>>> "operation" is vague, what exactly does it include? Also what does
>>>> "operate its device status" mean?
>>> Because we need to resume the device after suspending it by setting
>>> DRIVER_OK, so we need the device status
>>> filed keel alive.
>> This doesn't answer what "Pause its operation" means. If the
>> specification is not precise, then the best that can be implemented is
>> for a specific device to be compatible with a specific driver. If we
>> actually want portable, spec-compliant devices, we need explicitly
>> defined MUST/MUST NOT requirements. Presumably we want something like:
>>
>> A device MUST not send notifications while suspended.
>> A device MUST ignore writes to its device configuration while
>> suspended, except for writes to the device status byte.
>> A driver MUST NOT write to a suspended device's configuration space,
>> except for the device status byte.
>>
>> We also need to specify what a suspended device is allowed to do with
>> buffers it owns. For my use case of just suspending/resuming a VM, I
>> don't think it particularly matters if a suspended device writes to
>> buffers or descriptors while it is suspended. But based on what you
>> wrote about finishing any in-flight processing of buffers and not
>> processing any empty buffers, presumably you would prefer that the
>> device not be allowed to access buffer/descriptors while it is
>> suspended? That approach is probably cleaner from a pure specification
>> standpoint, but it's also more restrictive for devices and thus harder
>> to properly implement.
>>
>> -David
> Of course it's a bit harder. It does not sound too restrictive
> to me: as driver will not be adding new buffers, if device is
> still processing queues just defer setting the bit until it does not.
Yes, I agree

Thanks
Zhu Lingshan
>


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