arm64: mm: Permit transitioning from Global to Non-Global without BBM

Commit 4e602056 upstream.

Break-before-make is not needed when transitioning from Global to
Non-Global mappings, provided that the contiguous hint is not being used.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/mm/mmu.c

@@ -117,6 +117,10 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
 	if ((old | new) & PTE_CONT)
 		return false;
 
+	/* Transitioning from Global to Non-Global is safe */
+	if (((old ^ new) == PTE_NG) && (new & PTE_NG))
+		return true;
+
 	return ((old ^ new) & ~mask) == 0;
 }