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

[Jason Gunthorpe]

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

> 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.

Jason