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

Tian, Kevin
2022-07-01T06:08:08Z

> From: Alexey Kardashevskiy <aik@ozlabs.ru>
> Sent: Friday, July 1, 2022 12:58 PM
>
> 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.

In this case below check is more reasonable to be put in type1
attach_group(). Do a iommu_group_for_each_dev() to verify
CACHE_COHERENCY similar to what Robin did for INTR_REMAP.

(sorry no access to my build machine now but I suppose Jason
can soon work out a fix once he sees this. 😊)

>
>
> >
> > 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