mm: Introduce new flag to indicate wc safe

The VM_ALLOW_ANY_UNCACHED flag is implemented for ARM64, allowing KVM
stage 2 device mapping attributes to use NormalNC rather than
DEVICE_nGnRE, which allows guest mappings supporting write-combining
attributes (WC). ARM does not architecturally guarantee this is safe,
and indeed some MMIO regions like the GICv2 VCPU interface can trigger
uncontained faults if NormalNC is used.

Even worse, the expectation is that there are platforms where even
DEVICE_nGnRE can allow uncontained faults in corner cases. Unfortunately
existing ARM IP requires platform integration to take responsibility to
prevent this.

To safely use VFIO in KVM the platform must guarantee full safety in the
guest where no action taken against a MMIO mapping can trigger an
uncontained failure. The assumption is that most VFIO PCI platforms
support this for both mapping types, at least in common flows, based
on some expectations of how PCI IP is integrated. This can be enabled
more broadly, for instance into vfio-platform drivers, but only after
the platform vendor completes auditing for safety.

The VMA flag VM_ALLOW_ANY_UNCACHED was found to be the simplest and
cleanest way to communicate the information from VFIO to KVM that
mapping the region in S2 as NormalNC is safe. KVM consumes it to
activate the code that does the S2 mapping as NormalNC.

Suggested-by: Catalin Marinas <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Marc Zyngier <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Signed-off-by: Ankit Agrawal <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Oliver Upton <[email protected]>

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