Re: [PATCH] vfio: Remove VFIO_TYPE1_NESTING_IOMMU

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, May 20, 2022 at 01:00:32PM +0100, Will Deacon wrote:
> On Fri, May 20, 2022 at 12:02:23PM +0100, Robin Murphy wrote:
> > On 2022-05-10 17:55, Jason Gunthorpe via iommu wrote:
> > > This control causes the ARM SMMU drivers to choose a stage 2
> > > implementation for the IO pagetable (vs the stage 1 usual default),
> > > however this choice has no visible impact to the VFIO user.
> > 
> > Oh, I should have read more carefully... this isn't entirely true. Stage 2
> > has a different permission model from stage 1, so although it's arguably
> > undocumented behaviour, VFIO users that know enough about the underlying
> > system could use this to get write-only mappings if they so wish.
> 
> There's also an impact on combining memory attributes, but it's not hugely
> clear how that impacts userspace via things like VFIO_DMA_CC_IOMMU.

VFIO_DMA_CC_IOMMU has been clarified now to only be about the ability
to reject device requested no-snoop transactions. ie ignore the
NoSnoop bit in PCIe TLPs.

AFAICT SMMU can't implement this currently, and maybe doesn't need to.

VFIO requires the IO page tables be setup for cache coherent operation
for ordinary device DMA otherwises users like DPDK will not work.

It is surprising to hear you say that VFIO_TYPE1_NESTING_IOMMU might
also cause the IO to become non-coherent..

Jason