Re: [PATCH 2/5] vfio: Require that devices support DMA cache coherence

[Alex Williamson <alex.williamson@redhat.com>]

On Tue,  5 Apr 2022 13:16:01 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> dev_is_dma_coherent() is the control to determine if IOMMU_CACHE can be
> supported.
> 
> 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 dev_is_dma_coherent() before allowing a device to join a
> domain. This will block device/platform/iommu combinations from using VFIO
> that do not support cache coherent DMA.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/vfio/vfio.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index a4555014bd1e72..2a3aa3e742d943 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -32,6 +32,7 @@
>  #include <linux/vfio.h>
>  #include <linux/wait.h>
>  #include <linux/sched/signal.h>
> +#include <linux/dma-map-ops.h>
>  #include "vfio.h"
>  
>  #define DRIVER_VERSION	"0.3"
> @@ -1348,6 +1349,11 @@ static int vfio_group_get_device_fd(struct vfio_group *group, char *buf)
>  	if (IS_ERR(device))
>  		return PTR_ERR(device);
>  
> +	if (group->type == VFIO_IOMMU && !dev_is_dma_coherent(device->dev)) {
> +		ret = -ENODEV;
> +		goto err_device_put;
> +	}
> +

Failing at the point where the user is trying to gain access to the
device seems a little late in the process and opaque, wouldn't we
rather have vfio bus drivers fail to probe such devices?  I'd expect
this to occur in the vfio_register_group_dev() path.  Thanks,

Alex

>  	if (!try_module_get(device->dev->driver->owner)) {
>  		ret = -ENODEV;
>  		goto err_device_put;