Re: [PATCH v3 03/15] hw/pci: Add a pci_device_iommu_memory_region() helper

[Peter Xu <peterx@redhat.com>]

[ I forgot to really copy anyone, as usual.. trying again ]

On Tue, Jun 06, 2023 at 11:03:11AM -0400, Peter Xu wrote:
> On Tue, Jun 06, 2023 at 12:22:16PM +0100, Joao Martins wrote:
> > On 05/06/2023 17:57, Peter Xu wrote:
> > > On Tue, May 30, 2023 at 06:59:25PM +0100, Joao Martins wrote:
> > >> Much like pci_device_iommu_address_space() fetches the IOMMU AS, add a
> > >> pci_device_iommu_memory_region() which lets it return an the IOMMU MR
> > >> associated with it. The IOMMU MR is returned correctly for vIOMMUs using
> > >> pci_setup_iommu_info(). Note that today most vIOMMUs create the address
> > >> space and IOMMU MR at the same time, it's just mainly that there's API
> > >> to make the latter available.
> > > 
> > > Have you looked into other archs outside x86?  IIRC on some other arch one
> > > address space can have >1 IOMMU memory regions.. at least with such AS and
> > > MR layering it seems always possible?  Thanks,
> > > 
> > 
> > I looked at all callers of pci_setup_iommu() restricting to those that actually
> > track an IOMMUMemoryRegion when they create a address space... as this is where
> > pci_device_iommu_memory_region() is applicable. From looking at those[*], I see
> > always a 1:1 association between the AS and the IOMMU-MR in their initialization
> > when iommu_fn is called. Unless I missed something... Is there an arch you were
> > thinking specifically?
> 
> If only observing the ones that "track an IOMMUMemoryRegion when they
> create a address space", probably we're fine.  I was thinking ppc but I
> don't really know the details, and I assume that's not in the scope.
> Copying David Gibson just in case he got some comments here.
> 
> > 
> > [I am not sure we can track today an 1:N AS->IOMMU association today in Qemu]
> 
> IIUC we can?  The address space only have a root MR, and with that after
> translate() upon the root mr (per address_space_translate_iommu(), it can
> even be a few rounds of nested translations) it can go into whatever MR
> under it IIUC.  Different ranges can map to a different IOMMU MR logically.
> 
> > 
> > [*] alpha, arm smmu, ppc, s390, virtio, and some pci bridges (pnv_phb3 and pnv_phb4)
> 
> I just worried what we need here is not a MR object but a higher level
> object like the vIOMMU object.  We used to have a requirement with Scalable
> IOV (SVA) on Intel.  I tried to dig a bit in my inbox, not sure whether
> it's the latest status, just to show what I meant:
> 
> https://lore.kernel.org/r/20210302203827.437645-6-yi.l.liu@intel.com
> 
> Copy Yi too for that too.  From that aspect it makes more sense to me to
> fetching things from either an IOMMUops or "an iommu object", rather than
> relying on a specific MR (it'll also make it even harder when we can have
> >1 vIOMMUs so different MR can point to different IOMMUs in the future).
> 
> I assume the two goals have similar requirement, iiuc.  If that's the case,
> we'd better make sure we'll have one way to work for both.
> 
> -- 
> Peter Xu

-- 
Peter Xu