Re: [PATCH 3/5] iommu/virtio: Move to domain_alloc_paging()

Jean-Philippe Brucker
2025-02-19T10:34:54Z

On Wed, Feb 19, 2025 at 05:39:19PM +0800, Yu Zhang wrote:
> On Thu, Feb 13, 2025 at 06:09:19PM +0000, Jean-Philippe Brucker wrote:
> > On Fri, Feb 14, 2025 at 01:03:43AM +0800, Yu Zhang wrote:
> > > On Thu, Feb 13, 2025 at 09:46:01AM +0000, Jean-Philippe Brucker wrote:
> > > > Hi Jacob,
> > > >
> > > > On Wed, Feb 12, 2025 at 09:47:23PM -0800, Jacob Pan wrote:
> > > > > Our code and backend support are still in the early stages, that is why
> > > > > I am attempting to convert virtio-iommu driver to iommu_pt. Not sure if
> > > > > anyone has done the QEMU part to support VIRTIO_IOMMU_F_ATTACH_TABLE?
> > > > > @Jean @Eric Do you know?
> > > >
> > > > As far as I know Tina worked on this most recently:
> > > > https://github.com/TinaZhangZW/qemu/commits/virtio-iommu/vt-d-pgtable/
> > > > https://lore.kernel.org/all/20231106071226.9656-1-tina.zhang@intel.com/
> > >
> > > Thanks a lot for this information, Jean.
> > > IIUC, these patches were trying to add VT-d IO page table support in
> > > virtio-iommu, but it is not based on Jason's generic PT [1]. Just wondering,
> > > does anyone have plan to do the incorporation?
> >
> > I'm not aware of anyone working on this at the moment. Something you will
> > need for a portable pviommu is a library that manages PASID tables rather
> > than page tables [1], because the Arm SMMUv3 arch only support assigning
> > PASID tables to the guest. Alternatively you could implement opaque PASID
> > table allocation via host calls, letting the guest allocate GPA space and
> > the host manage the PASID table, but that idea didn't seem very popular at
> > the time.
>
> Thank you, Jean. Just had a study of the spec. For ARM SMMUv3, letting
> the guest manage the PASID table, and then assigning it directly to the
> backend in ATTACH_TABLE request looks quite resonable. But for VT-d,
> my understanding is the PASID table shall be managed by host. By "that
> idea didn't seem very popular", do you mean that people also want the
> ATTCH_TABLE request for VT-d also assign the PASID table(an virtual one
> managed by the guest). If yes, why?

No, the proposal for managing the PASID table in the host was done before
the VT-d architecture added Scalable mode, so at the time they also had to
assign whole PASID tables to the guest and weren't keen on managing it in
the host. I believe in revision 3 (2018) the architecture added support
for Scalable mode and the ability to manage PASID tables in the host.

Nowadays it wouldn't make sense for a pvIOMMU to manage the VT-d PASID
tables in the guest, because as I understand it there is no demand for
supporting the legacy mode address translation of VT-d.

Thanks,
Jean