iommu/arm-smmu-v3: Use S2FWB for NESTED domains

Force Write Back (FWB) changes how the S2 IOPTE's MemAttr field
works. When S2FWB is supported and enabled the IOPTE will force cachable
access to IOMMU_CACHE memory when nesting with a S1 and deny cachable
access when !IOMMU_CACHE.

When using a single stage of translation, a simple S2 domain, it doesn't
change things for PCI devices as it is just a different encoding for the
existing mapping of the IOMMU protection flags to cachability attributes.
For non-PCI it also changes the combining rules when incoming transactions
have inconsistent attributes.

However, when used with a nested S1, FWB has the effect of preventing the
guest from choosing a MemAttr in it's S1 that would cause ordinary DMA to
bypass the cache. Consistent with KVM we wish to deny the guest the
ability to become incoherent with cached memory the hypervisor believes is
cachable so we don't have to flush it.

Allow NESTED domains to be created if the SMMU has S2FWB support and use
S2FWB for NESTING_PARENTS. This is an additional option to CANWBS.

Link: https://patch.msgid.link/r/10-v4-9e99b76f3518+3a8-smmuv3_nesting_jgg@nvidia.com


Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Donald Dutile <ddutile@redhat.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>