KVM: arm64: selftests: Explicitly set the page attrs to Inner-Shareable

Atomic instructions such as 'ldset' in the guest have been observed to
cause an EL1 data abort with FSC 0x35 (IMPLEMENTATION DEFINED fault
(Unsupported Exclusive or Atomic access)) on Neoverse-N3.

Per DDI0487L.a B2.2.6, atomic instructions are only architecturally
guaranteed for Inner/Outer Shareable Normal Write-Back memory. For
anything else the behavior is IMPLEMENTATION DEFINED and can lose
atomicity, or, in this case, generate an abort.

It would appear that selftests sets up the stage-1 mappings as Non
Shareable, leading to the observed abort. Explicitly set the
Shareability field to Inner Shareable for non-LPA2 page tables. Note
that for the LPA2 page table format, translations for cacheable memory
inherit the shareability attribute of the PTW, i.e. TCR_ELx.SH{0,1}.

Suggested-by: Oliver Upton <oupton@google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Link: https://lore.kernel.org/r/20250405001042.1470552-3-rananta@google.com
[oliver: Rephrase changelog]
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

tools/testing/selftests/kvm/include/arm64/processor.h

@@ -113,6 +113,7 @@
 #define PMD_TYPE_TABLE		BIT(1)
 #define PTE_TYPE_PAGE		BIT(1)
 
+#define PTE_SHARED		(UL(3) << 8) /* SH[1:0], inner shareable */
 #define PTE_AF			BIT(10)
 
 #define PTE_ADDR_MASK(page_shift)	GENMASK(47, (page_shift))

tools/testing/selftests/kvm/lib/arm64/processor.c

@@ -172,6 +172,9 @@ static void _virt_pg_map(struct kvm_vm *vm, uint64_t vaddr, uint64_t paddr,
 	}
 
 	pg_attr = PTE_AF | PTE_ATTRINDX(attr_idx) | PTE_TYPE_PAGE | PTE_VALID;
+	if (!use_lpa2_pte_format(vm))
+		pg_attr |= PTE_SHARED;
+
 	*ptep = addr_pte(vm, paddr, pg_attr);
 }