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

[Jean-Philippe Brucker]

On Thu, Oct 21, 2021 at 08:22:23PM -0300, Jason Gunthorpe wrote:
> On Thu, Oct 21, 2021 at 03:58:02PM +0100, Jean-Philippe Brucker wrote:
> > 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().
> 
> So what happens in a VM? Does a VM know that arch_sync_dma_for_cpu()
> is not available?

This would only affect userspace drivers, it's only host or guest
userspace that cannot issue the maintenance operations. The VM can do
arch_sync_dma_for_cpu()

Thanks,
Jean

> 
> And how does this work with the nested IOMMU translation? I thought I
> read in the SMMU spec that the io page table entries could control
> cachability including in nesting cases?
> 
> > 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?
> 
> No, the worst is the same as Intel - a driver running in the guest VM
> assumes it can use arch_sync_dma_for_cpu() and acts accordingly,
> resulting in a broken VM.
> 
> Jason