Re: [PATCH V4 5/6] vDPA: answer num of queue pairs = 1 to userspace when VIRTIO_NET_F_MQ == 0

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

On 8/9/2022 6:09 PM, Jason Wang wrote:
> On Wed, Aug 10, 2022 at 8:54 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
>>
>>
>> On 8/9/2022 12:36 PM, Michael S. Tsirkin wrote:
>>> On Fri, Jul 22, 2022 at 01:14:42PM +0000, Parav Pandit wrote:
>>>>> From: Zhu Lingshan <lingshan.zhu@intel.com>
>>>>> Sent: Friday, July 22, 2022 7:53 AM
>>>>>
>>>>> If VIRTIO_NET_F_MQ == 0, the virtio device should have one queue pair, so
>>>>> when userspace querying queue pair numbers, it should return mq=1 than
>>>>> zero.
>>>>>
>>>>> Function vdpa_dev_net_config_fill() fills the attributions of the vDPA
>>>>> devices, so that it should call vdpa_dev_net_mq_config_fill() so the
>>>>> parameter in vdpa_dev_net_mq_config_fill() should be feature_device than
>>>>> feature_driver for the vDPA devices themselves
>>>>>
>>>>> Before this change, when MQ = 0, iproute2 output:
>>>>> $vdpa dev config show vdpa0
>>>>> vdpa0: mac 00:e8:ca:11:be:05 link up link_announce false max_vq_pairs 0
>>>>> mtu 1500
>>>>>
>>>>> After applying this commit, when MQ = 0, iproute2 output:
>>>>> $vdpa dev config show vdpa0
>>>>> vdpa0: mac 00:e8:ca:11:be:05 link up link_announce false max_vq_pairs 1
>>>>> mtu 1500
>>>>>
>>>> No. We do not want to diverge returning values of config space for fields which are not present as discussed in previous versions.
>>>> Please drop this patch.
>>>> Nack on this patch.
>>> Wrt this patch as far as I'm concerned you guys are bikeshedding.
>>>
>>> Parav generally don't send nacks please they are not helpful.
>>>
>>> Zhu Lingshan please always address comments in some way.  E.g. refer to
>>> them in the commit log explaining the motivation and the alternatives.
>>> I know you don't agree with Parav but ghosting isn't nice.
>>>
>>> I still feel what we should have done is
>>> not return a value, as long as we do return it we might
>>> as well return something reasonable, not 0.
>> Maybe I missed something but I don't get this, when VIRTIO_NET_F_MQ is
>> not negotiated, the VDPA_ATTR_DEV_NET_CFG_MAX_VQP attribute is simply
>> not there, but userspace (iproute) mistakenly puts a zero value there.
>> This is a pattern every tool in iproute follows consistently by large. I
>> don't get why kernel has to return something without seeing a very
>> convincing use case?
>>
>> Not to mention spec doesn't give us explicit definition for when the
>> field in config space becomes valid and/or the default value at first
>> sights as part of feature negotiation. If we want to stick to the model
>> Lingshan proposed, maybe fix the spec first then get back on the details?
> So spec said
>
> "
> The following driver-read-only field, max_virtqueue_pairs only exists
> if VIRTIO_NET_F_MQ or VIRTIO_NET_F_RSS is set.
> "
>
> My understanding is that the field is always valid if the device
> offers the feature.
The tricky part is to deal with VERSION_1 on transitional device that 
determines the endianness of field. I know we don't support !VERSION_1 
vdpa provider for now, but the tool should be made independent of this 
assumption.

For the most of config fields there's no actual valid "default" value 
during feature negotiation until it can be determined after negotiation 
is done. I wonder what is the administrative value if presenting those 
random value to the end user? And there's even special feature like 
VIRTIO_BLK_F_CONFIG_WCE that only present valid feature value after 
negotiation. I'm afraid it may further confuse end user, or it would 
require them to read and understand all of details in spec, which 
apparently contradict to the goal of showing meaningful queue-pair value 
without requiring user to read the spec details.

-Siwei

>
> Btw, even if the spec is unclear, it would be very hard to "fix" it
> without introducing a new feature bit, it means we still need to deal
> with device without the new feature.
>
> Thanks
>
>> -Siwei
>>
>>> And I like it that this fixes sparse warning actually:
>>> max_virtqueue_pairs it tagged as __virtio, not __le.
>>>
>>> However, I am worried there is another bug here:
>>> this is checking driver features. But really max vqs
>>> should not depend on that, it depends on device
>>> features, no?
>>>
>>>
>>>
>>>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>>>>> ---
>>>>>    drivers/vdpa/vdpa.c | 7 ++++---
>>>>>    1 file changed, 4 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c index
>>>>> d76b22b2f7ae..846dd37f3549 100644
>>>>> --- a/drivers/vdpa/vdpa.c
>>>>> +++ b/drivers/vdpa/vdpa.c
>>>>> @@ -806,9 +806,10 @@ static int vdpa_dev_net_mq_config_fill(struct
>>>>> vdpa_device *vdev,
>>>>>      u16 val_u16;
>>>>>
>>>>>      if ((features & BIT_ULL(VIRTIO_NET_F_MQ)) == 0)
>>>>> -           return 0;
>>>>> +           val_u16 = 1;
>>>>> +   else
>>>>> +           val_u16 = __virtio16_to_cpu(true, config-
>>>>>> max_virtqueue_pairs);
>>>>> -   val_u16 = le16_to_cpu(config->max_virtqueue_pairs);
>>>>>      return nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP,
>>>>> val_u16);  }
>>>>>
>>>>> @@ -842,7 +843,7 @@ static int vdpa_dev_net_config_fill(struct
>>>>> vdpa_device *vdev, struct sk_buff *ms
>>>>>                            VDPA_ATTR_PAD))
>>>>>              return -EMSGSIZE;
>>>>>
>>>>> -   return vdpa_dev_net_mq_config_fill(vdev, msg, features_driver,
>>>>> &config);
>>>>> +   return vdpa_dev_net_mq_config_fill(vdev, msg, features_device,
>>>>> +&config);
>>>>>    }
>>>>>
>>>>>    static int
>>>>> --
>>>>> 2.31.1
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