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

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Date:   Wed, 28 Apr 2021 18:20:10 +0800
From:   "Zhu, Lingshan" <lingshan.zhu@...el.com>
To:     Jason Wang <jasowang@...hat.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



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.

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)
>>>>>
>>>>
>>>
>>
>