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

[Zhu, Lingshan]

On 4/28/2021 9:08 PM, Jason Wang wrote:
> 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@xxxxxxxxx>
> > > > > > > > ---
> > > > > > > >   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).
For the page alignment checks, I think this is the point of using 
PAGE_SIZE, we want the doorbell placed at the page boundary, PAGE_SIZE 
depends on the arch,
so I think we don't want to use hard code here. We will pass the 
notify_pa to upper layer anyway, just print an warning if not PAGE_SIZE 
aligned.

However I think this may refer to vdpa_notification_area.size, YES, I 
think use PAGE_SIZE directly is wrong here, this size depends on the 
device(bar layout) than the arch, so I will add more code to tell which 
device is probed by the driver, then assign correct value.

Thanks
> 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)