Re: [PATCH 1/4] vdpa: Add stop operation

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 5/23/2022 12:20 PM, Eugenio Perez Martin wrote:
> On Sat, May 21, 2022 at 12:13 PM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>>
>>
>> On 5/20/2022 10:23 AM, Eugenio Pérez wrote:
>>> This operation is optional: It it's not implemented, backend feature bit
>>> will not be exposed.
>>>
>>> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
>>> ---
>>>    include/linux/vdpa.h | 6 ++++++
>>>    1 file changed, 6 insertions(+)
>>>
>>> diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
>>> index 15af802d41c4..ddfebc4e1e01 100644
>>> --- a/include/linux/vdpa.h
>>> +++ b/include/linux/vdpa.h
>>> @@ -215,6 +215,11 @@ struct vdpa_map_file {
>>>     * @reset:                  Reset device
>>>     *                          @vdev: vdpa device
>>>     *                          Returns integer: success (0) or error (< 0)
>>> + * @stop:                    Stop or resume the device (optional, but it must
>>> + *                           be implemented if require device stop)
>>> + *                           @vdev: vdpa device
>>> + *                           @stop: stop (true), not stop (false)
>>> + *                           Returns integer: success (0) or error (< 0)
>> Is this uAPI meant to address all use cases described in the full blown
>> _F_STOP virtio spec proposal, such as:
>>
>> --------------%<--------------
>>
>> ...... the device MUST finish any in flight
>> operations after the driver writes STOP.  Depending on the device, it
>> can do it
>> in many ways as long as the driver can recover its normal operation if it
>> resumes the device without the need of resetting it:
>>
>> - Drain and wait for the completion of all pending requests until a
>>     convenient avail descriptor. Ignore any other posterior descriptor.
>> - Return a device-specific failure for these descriptors, so the driver
>>     can choose to retry or to cancel them.
>> - Mark them as done even if they are not, if the kind of device can
>>     assume to lose them.
>> --------------%<--------------
>>
> Right, this is totally underspecified in this series.
>
> I'll expand on it in the next version, but that text proposed to
> virtio-comment was complicated and misleading. I find better to get
> the previous version description. Would the next description work?
>
> ```
> After the return of ioctl, the device MUST finish any pending operations like
> in flight requests. It must also preserve all the necessary state (the
> virtqueue vring base plus the possible device specific states)
Hmmm, "possible device specific states" is a bit vague. Does it require 
the device to save any device internal state that is not defined in the 
virtio spec - such as any failed in-flight requests to resubmit upon 
resume? Or you would lean on SVQ to intercept it in depth and save it 
with some other means? I think network device also has internal state 
such as flow steering state that needs bookkeeping as well.

A follow-up question is what is the use of the `stop` argument of false, 
does it require the device to support resume? I seem to recall this is 
something to abandon in favor of device reset plus setting queue 
base/addr after. Or it's just a optional feature that may be device 
specific (if one can do so in simple way).

-Siwei

>   that is required
> for restoring in the future.
>
> In the future, we will provide features similar to VHOST_USER_GET_INFLIGHT_FD
> so the device can save pending operations.
> ```
>
> Thanks for pointing it out!
>
>
>
>
>
>> E.g. do I assume correctly all in flight requests are flushed after
>> return from this uAPI call? Or some of pending requests may be subject
>> to loss or failure? How does the caller/user specify these various
>> options (if there are) for device stop?
>>
>> BTW, it would be nice to add the corresponding support to vdpa_sim_blk
>> as well to demo the stop handling. To just show it on vdpa-sim-net IMHO
>> is perhaps not so convincing.
>>
>> -Siwei
>>
>>>     * @get_config_size:                Get the size of the configuration space includes
>>>     *                          fields that are conditional on feature bits.
>>>     *                          @vdev: vdpa device
>>> @@ -316,6 +321,7 @@ struct vdpa_config_ops {
>>>        u8 (*get_status)(struct vdpa_device *vdev);
>>>        void (*set_status)(struct vdpa_device *vdev, u8 status);
>>>        int (*reset)(struct vdpa_device *vdev);
>>> +     int (*stop)(struct vdpa_device *vdev, bool stop);
>>>        size_t (*get_config_size)(struct vdpa_device *vdev);
>>>        void (*get_config)(struct vdpa_device *vdev, unsigned int offset,
>>>                           void *buf, unsigned int len);