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

From: Jason Gunthorpe <[email-masked]>
Date: Tue, 5 Apr 2022 16:29:16 -0300

On Tue, Apr 05, 2022 at 01:10:44PM -0600, Alex Williamson wrote:
> On Tue,  5 Apr 2022 13:16:01 -0300
> Jason Gunthorpe <[email-masked]> 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 <[email-masked]>
> >  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