netdev - Re: [PATCH 2/2] vDPA/ifcvf: implement doorbell mapping for ifcvf

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Date:   Wed, 28 Apr 2021 21:08:35 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>, mst@...hat.com
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] vDPA/ifcvf: implement doorbell mapping for ifcvf


在 2021/4/28 下午6:20, Zhu, Lingshan 写道:
>
>
> On 4/28/2021 6:03 PM, Jason Wang wrote:
>>
>> 在 2021/4/28 下午5:56, Zhu, Lingshan 写道:
>>>
>>>
>>> On 4/28/2021 5:21 PM, Jason Wang wrote:
>>>>
>>>> 在 2021/4/28 下午4:59, Zhu, Lingshan 写道:
>>>>>
>>>>>
>>>>> On 4/28/2021 4:42 PM, Jason Wang wrote:
>>>>>>
>>>>>> 在 2021/4/28 下午4:21, Zhu Lingshan 写道:
>>>>>>> This commit implements doorbell mapping feature for ifcvf.
>>>>>>> This feature maps the notify page to userspace, to eliminate
>>>>>>> vmexit when kick a vq.
>>>>>>>
>>>>>>> Signed-off-by: Zhu Lingshan <lingshan.zhu@...el.com>
>>>>>>> ---
>>>>>>>   drivers/vdpa/ifcvf/ifcvf_main.c | 18 ++++++++++++++++++
>>>>>>>   1 file changed, 18 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/vdpa/ifcvf/ifcvf_main.c 
>>>>>>> b/drivers/vdpa/ifcvf/ifcvf_main.c
>>>>>>> index e48e6b74fe2e..afcb71bc0f51 100644
>>>>>>> --- a/drivers/vdpa/ifcvf/ifcvf_main.c
>>>>>>> +++ b/drivers/vdpa/ifcvf/ifcvf_main.c
>>>>>>> @@ -413,6 +413,23 @@ static int ifcvf_vdpa_get_vq_irq(struct 
>>>>>>> vdpa_device *vdpa_dev,
>>>>>>>       return vf->vring[qid].irq;
>>>>>>>   }
>>>>>>>   +static struct vdpa_notification_area 
>>>>>>> ifcvf_get_vq_notification(struct vdpa_device *vdpa_dev,
>>>>>>> +                                   u16 idx)
>>>>>>> +{
>>>>>>> +    struct ifcvf_hw *vf = vdpa_to_vf(vdpa_dev);
>>>>>>> +    struct vdpa_notification_area area;
>>>>>>> +
>>>>>>> +    if (vf->notify_pa % PAGE_SIZE) {
>>>>>>> +        area.addr = 0;
>>>>>>> +        area.size = 0;
>>>>>>
>>>>>>
>>>>>> We don't need this since:
>>>>>>
>>>>>> 1) there's a check in the vhost vDPA
>>>>> I think you mean this code block in vdpa.c
>>>>>         notify = ops->get_vq_notification(vdpa, index);
>>>>>         if (notify.addr & (PAGE_SIZE - 1))
>>>>>                 return -EINVAL;
>>>>>
>>>>> This should work, however, I think the parent driver should ensure 
>>>>> it passes a PAGE_SIZE aligned address to userspace, to be robust, 
>>>>> to be reliable.
>>>>
>>>>
>>>> The point is parent is unaware of whether or not there's a userspace.
>>> when calling this, I think it targets a usersapce program, why 
>>> kernel space need it, so IMHO no harm if we check this to keep the 
>>> parent driver robust.
>>
>>
>> Again, vDPA device is unaware of what driver that is bound. It could 
>> be virtio-vpda, vhost-vdpa or other in the future. It's only the vDPA 
>> bus driver know how it is actually used.
>>
>>
>>>>
>>>>
>>>>>> 2) device is unaware of the bound driver, non page aligned 
>>>>>> doorbell doesn't necessarily meant it can be used
>>>>> Yes, non page aligned doorbell can not be used, so there is a check.
>>>>
>>>>
>>>> Typo, what I meant is "it can't be used". That is to say, we should 
>>>> let the vDPA bus driver to decide whether or not it can be used.
>>> If it is not page aligned, there would be extra complexities for 
>>> vhost/qemu, I see it as a hardware defect, 
>>
>>
>> It is allowed by the virtio spec, isn't it?
> The spec does not require the doorbell to be page size aligned, 
> however it still a hardware defect if non page size aligned notify 
> base present, I will leave a warning message here instead of the 0 value.
>

Another note is that, using PAGE_SIZE is wrong here since it varies 
among archs (at most 64K on some one).

Thanks


> Thanks
> Zhu Lingshan
>>
>> Thanks
>>
>>
>>> why adapt to this kind of defects?
>>>
>>> Thanks
>>> Zhu Lingshan
>>>>
>>>> Thanks
>>>>
>>>>
>>>>>
>>>>> Thanks
>>>>> Zhu Lingshan
>>>>>>
>>>>>> Let's leave those polices to the driver.
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>>
>>>>>>> +    } else {
>>>>>>> +        area.addr = vf->notify_pa;
>>>>>>> +        area.size = PAGE_SIZE;
>>>>>>> +    }
>>>>>>> +
>>>>>>> +    return area;
>>>>>>> +}
>>>>>>> +
>>>>>>>   /*
>>>>>>>    * IFCVF currently does't have on-chip IOMMU, so not
>>>>>>>    * implemented set_map()/dma_map()/dma_unmap()
>>>>>>> @@ -440,6 +457,7 @@ static const struct vdpa_config_ops 
>>>>>>> ifc_vdpa_ops ={
>>>>>>>       .get_config    = ifcvf_vdpa_get_config,
>>>>>>>       .set_config    = ifcvf_vdpa_set_config,
>>>>>>>       .set_config_cb  = ifcvf_vdpa_set_config_cb,
>>>>>>> +    .get_vq_notification = ifcvf_get_vq_notification,
>>>>>>>   };
>>>>>>>     static int ifcvf_probe(struct pci_dev *pdev, const struct 
>>>>>>> pci_device_id *id)
>>>>>>
>>>>>
>>>>
>>>
>>
>