Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

[Jason Gunthorpe]

On Mon, Jun 10, 2024 at 04:04:30PM -0700, Nicolin Chen wrote:

> > > Actually, even now as we put a dispatcher in VMM, VMM still does
> > > decode the CD table to link ASID to s1_hwpt. Otherwise, it could
> > > only broadcast a TLBI cmd to all pSMMUs.
> > 
> > No, there should be no CD table decoding and no linking ASID to
> > anything by the VMM.
> > 
> > The ARM architecture is clean, the ASID can remain private to the VM,
> > there is no reason for the VMM to understand it.
> 
> But a guest-level TLBI command usually has only ASID available to
> know which pSMMU to dispatch the command. Without an ASID lookup
> table, how could VMM then dispatch a command to the corresponding
> pSMMU?

It can broadcast. The ARM architecture does not expect a N:1 mapping
of SMMUs. This is why I think it is not such a good idea..

Yes the VMM could walk the CD tables too and build up a bitmap of what
ASIDs are being used by what pSMMUs, and that would be fine for the
VMM to do, but I wouldn't necessarily recommend it :)

Jason