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

Alexey Kardashevskiy
2022-07-01T04:57:44Z

On 4/8/22 01:23, Jason Gunthorpe via iommu wrote:
> 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.

This just broke VFIO on POWER which does not use iommu_ops.

>
> 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;
> +
> return __vfio_register_dev(device,
> vfio_group_find_or_alloc(device->dev));
> }

--
Alexey