RE: [PATCH v2 4/4] vfio: Require that devices support DMA cache coherence

Tian, Kevin
2022-04-08T08:26:32Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, April 7, 2022 11:24 PM
>
> IOMMU_CACHE means that normal DMAs do not require any additional
> coherency
> mechanism and is the basic uAPI that VFIO exposes to userspace. For
> instance VFIO applications like DPDK will not work if additional coherency
> operations are required.
>
> Therefore check IOMMU_CAP_CACHE_COHERENCY like vdpa & usnic do
> before
> allowing an IOMMU backed VFIO device to be created.
>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
> drivers/vfio/vfio.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index a4555014bd1e72..9edad767cfdad3 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -815,6 +815,13 @@ static int __vfio_register_dev(struct vfio_device
> *device,
>
> int vfio_register_group_dev(struct vfio_device *device)
> {
> + /*
> + * VFIO always sets IOMMU_CACHE because we offer no way for
> userspace to
> + * restore cache coherency.
> + */
> + if (!iommu_capable(device->dev->bus,
> IOMMU_CAP_CACHE_COHERENCY))
> + return -EINVAL;
> +

One nit. Is it logistically more reasonable to put this patch before
changing VFIO to always set IOMMU_CACHE?

otherwise,

Reviewed-by: Kevin Tian <kevin.tian@intel.com>