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

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Date:   Wed, 28 Apr 2021 18:03:34 +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?

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)
>>>>
>>>
>>
>