Re: [PATCH v5 0/4] vfio: type1: support for ARM SMMUS with VFIO_IOMMU_TYPE1

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

On Fri, 2015-03-06 at 10:41 +0000, Will Deacon wrote:
> > As a longer term solution, would it make sense to add a user flag at VFIO
> > user API level to turn the IOMMU_CACHE on?
> 
> I think userspace certainly needs a way to figure out if a device is
> coherent or not, otherwise it can't generate the correct device-tree
> properties for something like a KVM guest but the IOMMU_* setting should
> remain in the kernel IMO. Similarly for things like MSI pages, which would
> need to be mapped as device memory on ARM -- that should be exposed as a
> higher level "please map my MSI page here" ioctl as opposed to requiring
> userspace to supply the correct memory attributes.

Userspace already has a way to determine if all the IOMMUs for a
container support cache coherence, as does KVM via the kvm-vfio pseudo
device.  It's inadvisable to allow userspace to control whether
IOMMU_CACHE is used on x86 as emulating cache sync instructions hurts
the overall system performance.  We don't want users to arbitrarily
decide to enable those heavy-weight operations.  Thanks,

Alex