Re: [RFC PATCH v4 03/16] iommu/arm-smmu-v3: Add initial pSMMU realm viommu plumbing

[Nicolin Chen <nicolinc@nvidia.com>]

On Tue, Sep 01, 2026 at 02:47:04PM +0530, Aneesh Kumar K.V wrote:
> Nicolin Chen <nicolinc@nvidia.com> writes:
> > But, on the other hand, I can imagine that a Realm VSMMU would add
> > a new flag with a user_data to this VIOMMU. Then, this flow would
> > give some troubles to VMM (QEMU for example):
> >
> >  - For VM with a guest-level SMMU, QEMU creates a realm instance
> >    where IOMMU_VIOMMU_TYPE_ARM_REALM_SMMUV3 (with vsmmu) can be
> >    allocated.
> >  - For VM w/o a guest-level SMMU, QEMU won't create such a realm
> >    instance, while still required to invoke the ioctl (w/o vsmmu). 
> >
> > Taking a step back, I wonder if we really need to use iommufd for
> > PSMMU activation and its stream table allocations?
> >
> > Here are some facts:
> >   - An iommufd has a ctx, that's one per VM. Similarly, a Realm
> >     has an RD.
> >   - For an RMI command that needs an RD, it makes sense to be per
> >     iommufd ctx, e.g. RMI_VSMMU_* or RMI_VDEV_* commands.
> >   - PSMMU commands are global; they don't need RD. So they don't
> >     seem necessary to tie to an iommufd ctx.
> >
> > Instead, could the PSMMU activation be done after RMI_PSMMU_INFO
> > check? Is there any reason not to do that? A safer timing might
> > be at the device assignment stage?
> >
> 
> One of the reasons I added IOMMU_VIOMMU_TYPE_ARM_REALM_SMMUV3 was to
> avoid creating a psmmu object when we are not using a PCI passthrough
> VM. That is also the reason for all the refcounting around the psmmu
> objects.
> 
> If we are okay with creating psmmu objects early, then I guess we can go
> with the above approach.

Do you see any trouble if we implement in this way:
 - Add "struct arm_smmu_realm *psmmu" in struct arm_smmu_device
 - RMI_PSMMU_INFO in probe():
   + allocate a smmu->psmmu on success
   + keep smmu->psmmu->users = 0; psmmu is not activated
 - Device binds to iommufd, creating idev
   + invoke an iommu op to a driver-level callback:
	psmmu = smmu->psmmu;
	if (!psmmu)
		return 0;
	guard(mutex)(&psmmu->mutex);
	if (!smmu->psmmu->users)
		rmi_psmmu_activate();
	psmmu->users++;
	rmi_psmmu_st_l2_create();
?

Nicolin