Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

[Jean-Philippe Brucker]

On Thu, Oct 21, 2021 at 02:26:00AM +0000, Tian, Kevin wrote:
> > I'll leave it to Jean to confirm. If only coherent DMA can be used in
> > the guest on other platforms, suppose VFIO should not blindly set
> > IOMMU_CACHE and in concept it should deny assigning a non-coherent
> > device since no co-ordination with guest exists today.
> 
> Jean, what's your opinion?

Yes a sanity check to prevent assigning non-coherent devices would be
good, though I'm not particularly worried about non-coherent devices. PCIe
on Arm should be coherent (according to the Base System Architecture). So
vfio-pci devices should be coherent, but vfio-platform and mdev are
case-by-case (hopefully all coherent since it concerns newer platforms).

More worrying, I thought we disabled No-Snoop for VFIO but I was wrong,
it's left enabled. On Arm I don't think userspace can perform the right
cache maintenance operations to maintain coherency with a device that
issues No-Snoop writes. Userspace can issue clean+invalidate but not
invalidate alone, so there is no equivalent to arch_sync_dma_for_cpu().
I think the worse that can happen is the device owner shooting itself in
the foot by using No-Snoop, but would it hurt to disable it?

Thanks,
Jean