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

[Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>]

On Fri, Apr 08, 2022 at 08:26:10AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, April 7, 2022 11:24 PM
> > 
> > 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.
> > 
> > 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
> > +++ 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;
> > +
> 
> One nit. Is it logistically more reasonable to put this patch before
> changing VFIO to always set IOMMU_CACHE?

For bisectability it has to be after

    iommu: Redefine IOMMU_CAP_CACHE_COHERENCY as the cap flag for IOMMU_CACHE

Otherwise Intel iommu will stop working with VFIO

The ordering is OK as is because no IOMMU that works with VFIO cares
about IOMMU_CACHE.

Jason
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