Re: userspace emulated smmu/vfio integration: how to trap updates to the table structures?

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 02, 2017 at 09:30:39AM +0200, Auger Eric wrote:
> Hi Will, Robin, Jean-Philippe,
> 
> I have been working on the integration between user-space emulated
> SMMU-v3 and VFIO in QEMU. At the moment I fail identifying a proper easy
> way to trap page table updates. This is requested to keep the host
> translation structures consistent to guest translation structures.
> 
> On Intel VTD there is a so-called "caching mode" (CM, see VTD spec
> paragraph 6.1) that forces the OS to explicitly invalidate caches
> whenever it updates any remapping structure (updates to not-present or
> present entries). Those invalidation commands are used to trap and
> update host structures. This mode was devised for virtualization. I was
> not able to find such "caching mode" on ARM SMMU. Is there any?
> 
> If not, do you have any other suggestion, I mean, besides the
> virtio-based solution.
> 
> Thanks
> 
> Eric

Does SMMU hardware have nested page table support?
Some IOMMUs have this.

If yes then for some workloads this has the potential to perform better
than the invalidation hack with host SMMU performing nested translations
using guest page tables.

-- 
MST