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

Jason Gunthorpe
2022-04-05T22:40:00Z

On Tue, Apr 05, 2022 at 01:10:44PM -0600, Alex Williamson wrote:
> 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
> > +++ 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,

Yes, that is a good point.

So like this:

int vfio_register_group_dev(struct vfio_device *device)
{
+ if (!dev_is_dma_coherent(device->dev))
+ return -EINVAL;
+
return __vfio_register_dev(device,
vfio_group_find_or_alloc(device->dev));
}

I fixed it up.

Thanks,
Jason