netdev - Re: [PATCH V3 6/6] vDPA: fix 'cast to restricted le16' warnings in vdpa.c

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Date:   Fri, 29 Jul 2022 18:18:40 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, parav@...dia.com, xieyongji@...edance.com,
        gautam.dawar@....com
Subject: Re: [PATCH V3 6/6] vDPA: fix 'cast to restricted le16' warnings in
 vdpa.c



On 7/29/2022 6:16 PM, Michael S. Tsirkin wrote:
> On Fri, Jul 29, 2022 at 06:01:38PM +0800, Zhu, Lingshan wrote:
>>
>> On 7/29/2022 5:39 PM, Michael S. Tsirkin wrote:
>>> On Fri, Jul 29, 2022 at 05:35:09PM +0800, Zhu, Lingshan wrote:
>>>> On 7/29/2022 5:23 PM, Michael S. Tsirkin wrote:
>>>>> On Fri, Jul 29, 2022 at 05:20:17PM +0800, Zhu, Lingshan wrote:
>>>>>> On 7/29/2022 5:17 PM, Michael S. Tsirkin wrote:
>>>>>>> On Fri, Jul 29, 2022 at 05:07:11PM +0800, Zhu, Lingshan wrote:
>>>>>>>> On 7/29/2022 4:53 PM, Michael S. Tsirkin wrote:
>>>>>>>>> On Fri, Jul 01, 2022 at 09:28:26PM +0800, Zhu Lingshan wrote:
>>>>>>>>>> This commit fixes spars warnings: cast to restricted __le16
>>>>>>>>>> in function vdpa_dev_net_config_fill() and
>>>>>>>>>> vdpa_fill_stats_rec()
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@...el.com>
>>>>>>>>>> ---
>>>>>>>>>>       drivers/vdpa/vdpa.c | 6 +++---
>>>>>>>>>>       1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
>>>>>>>>>> index 846dd37f3549..ed49fe46a79e 100644
>>>>>>>>>> --- a/drivers/vdpa/vdpa.c
>>>>>>>>>> +++ b/drivers/vdpa/vdpa.c
>>>>>>>>>> @@ -825,11 +825,11 @@ static int vdpa_dev_net_config_fill(struct vdpa_device *vdev, struct sk_buff *ms
>>>>>>>>>>       		    config.mac))
>>>>>>>>>>       		return -EMSGSIZE;
>>>>>>>>>> -	val_u16 = le16_to_cpu(config.status);
>>>>>>>>>> +	val_u16 = __virtio16_to_cpu(true, config.status);
>>>>>>>>>>       	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_STATUS, val_u16))
>>>>>>>>>>       		return -EMSGSIZE;
>>>>>>>>>> -	val_u16 = le16_to_cpu(config.mtu);
>>>>>>>>>> +	val_u16 = __virtio16_to_cpu(true, config.mtu);
>>>>>>>>>>       	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MTU, val_u16))
>>>>>>>>>>       		return -EMSGSIZE;
>>>>>>>>> Wrong on BE platforms with legacy interface, isn't it?
>>>>>>>>> We generally don't handle legacy properly in VDPA so it's
>>>>>>>>> not a huge deal, but maybe add a comment at least?
>>>>>>>> Sure, I can add a comment here: this is for modern devices only.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Zhu Lingshan
>>>>>>> Hmm. what "this" is for modern devices only here?
>>>>>> this cast, for LE modern devices.
>>>>> I think status existed in legacy for sure, and it's possible that
>>>>> some legacy devices backported mtu and max_virtqueue_pairs otherwise
>>>>> we would have these fields as __le not as __virtio, right?
>>>> yes, that's the reason why it is virtio_16 than just le16.
>>>>
>>>> I may find a better solution to detect whether it is LE, or BE without a
>>>> virtio_dev structure.
>>>> Check whether vdpa_device->get_device_features() has VIRTIO_F_VERISON_1. If
>>>> the device offers _F_VERSION_1, then it is a LE device,
>>>> or it is a BE device, then we use __virtio16_to_cpu(false, config.status).
>>>>
>>>> Does this look good?
>>> No since the question is can be a legacy driver with a transitional
>>> device.  I don't have a good idea yet. vhost has VHOST_SET_VRING_ENDIAN
>>> and maybe we need something like this for config as well?
>> Is it a little overkill to implementing vdpa_ops.get_endian()?
> I think the question is driver endian-ness.
>
> But another approach is really just to say userspace should
> tweak config endian itself.  Let's just say that in the comment?
> /*
>   * Assume little endian for now, userspace can tweak this for
>   * legacy guest support.
>   */
> ?
Oh, yes, the user space can tweak the value!!!

I will add this comment, thanks!!!!
>>>>>>>>>> @@ -911,7 +911,7 @@ static int vdpa_fill_stats_rec(struct vdpa_device *vdev, struct sk_buff *msg,
>>>>>>>>>>       	}
>>>>>>>>>>       	vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
>>>>>>>>>> -	max_vqp = le16_to_cpu(config.max_virtqueue_pairs);
>>>>>>>>>> +	max_vqp = __virtio16_to_cpu(true, config.max_virtqueue_pairs);
>>>>>>>>>>       	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP, max_vqp))
>>>>>>>>>>       		return -EMSGSIZE;
>>>>>>>>>> -- 
>>>>>>>>>> 2.31.1