RE: [PATCH v4 2/9] vfio-iommufd: Create iommufd_access for noiommu devices

Liu, Yi L
2023-04-27T06:59:32Z

> From: Tian, Kevin <kevin.tian@intel.com>
> Sent: Thursday, April 27, 2023 2:39 PM
>
> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Wednesday, April 26, 2023 10:54 PM
> > @@ -121,7 +128,8 @@ static void vfio_emulated_unmap(void *data,
> > unsigned long iova,
> > {
> > struct vfio_device *vdev = data;
> >
> > - if (vdev->ops->dma_unmap)
> > + /* noiommu devices cannot do map/unmap */
> > + if (vdev->noiommu && vdev->ops->dma_unmap)
> > vdev->ops->dma_unmap(vdev, iova, length);
>
> Is it necessary? All mdev devices implementing @dma_unmap won't
> set noiommu flag.

Hmmm. Yes, and all the devices set noiommu is not implementing @dma_unmap
as far as I see. Maybe this noiommu check can be removed.

>
> Instead in the future if we allow noiommu userspace to pin pages
> we'd need similar logic too.

I'm not quite sure about it so far. For mdev devices, the device driver
may use vfio_pin_pages/vfio_dma_rw () to pin page. Hence such drivers
need to listen to dma_unmap() event. But for noiommu users, does the
device driver also participate in the page pin? At least for vfio-pci driver,
it does not, or maybe it will in the future when enabling noiommu
userspace to pin pages. It looks to me such userspace should order
the DMA before calling ioctl to unpin page instead of letting device
driver listen to unmap.

Regards,
Yi Liu