Re: [PATCH v2 04/19] iommufd: Allow pt_id to carry viommu_id for IOMMU_HWPT_ALLOC

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Sep 27, 2024 at 08:59:25AM -0300, Jason Gunthorpe wrote:
> On Thu, Sep 26, 2024 at 11:02:37PM -0700, Nicolin Chen wrote:
> > On Fri, Sep 27, 2024 at 01:38:08PM +0800, Yi Liu wrote:
> > > > > Does it mean each vIOMMU of VM can only have
> > > > > one s2 HWPT?
> > > > 
> > > > Giving some examples here:
> > > >   - If a VM has 1 vIOMMU, there will be 1 vIOMMU object in the
> > > >     kernel holding one S2 HWPT.
> > > >   - If a VM has 2 vIOMMUs, there will be 2 vIOMMU objects in the
> > > >     kernel that can hold two different S2 HWPTs, or share one S2
> > > >     HWPT (saving memory).
> > > 
> > > So if you have two devices assigned to a VM, then you may have two
> > > vIOMMUs or one vIOMMU exposed to guest. This depends on whether the two
> > > devices are behind the same physical IOMMU. If it's two vIOMMUs, the two
> > > can share the s2 hwpt if their physical IOMMU is compatible. is it?
> > 
> > Yes.
> > 
> > > To achieve the above, you need to know if the physical IOMMUs of the
> > > assigned devices, hence be able to tell if physical IOMMUs are the
> > > same and if they are compatible. How would userspace know such infos?
> > 
> > My draft implementation with QEMU does something like this:
> >  - List all viommu-matched iommu nodes under /sys/class/iommu: LINKs
> >  - Get PCI device's /sys/bus/pci/devices/0000:00:00.0/iommu: LINK0
> >  - Compare the LINK0 against the LINKs
> > 
> > We so far don't have an ID for physical IOMMU instance, which can
> > be an alternative to return via the hw_info call, otherwise.
> 
> We could return the sys/class/iommu string from some get_info or
> something

I had a patch doing an ida alloc for each iommu_dev and returning
the ID via hw_info. It wasn't useful at that time, as we went for
fail-n-retry for S2 HWPT allocations on multi-pIOMMU platforms.

Perhaps that could be cleaner than returning a string?

> > For compatibility to share a stage-2 HWPT, basically we would do
> > a device attach to one of the stage-2 HWPT from the list that VMM
> > should keep. This attach has all the compatibility test, down to
> > the IOMMU driver. If it fails, just allocate a new stage-2 HWPT.
> 
> Ideally just creating the viommu should validate the passed in hwpt is
> compatible without attaching.

I think I should add a validation between hwpt->domain->owner and
dev_iommu_ops(idev->dev) then!

Thanks
Nicolin