[PATCH] KVM: arm64: selftests: Avoid testing the IMPDEF behavior

[Zenghui Yu <zenghui.yu@linux.dev>]

From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>

It turned out that we can't really force KVM to use the "slow" path when
emulating AT instructions [1]. We should therefore avoid testing the IMPDEF
behavior (i.e., TEST_ACCESS_FLAG - address translation instructions are
permitted to update AF but not required).

Remove it and improve the comment a bit.

[1] https://lore.kernel.org/r/b951dcfb-0ad1-4d7b-b6ce-d54b272dd9be@linux.dev

Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
---
 tools/testing/selftests/kvm/arm64/at.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/kvm/arm64/at.c b/tools/testing/selftests/kvm/arm64/at.c
index c8ee6f520734..ce5d312ef6ba 100644
--- a/tools/testing/selftests/kvm/arm64/at.c
+++ b/tools/testing/selftests/kvm/arm64/at.c
@@ -13,7 +13,6 @@
 
 enum {
 	CLEAR_ACCESS_FLAG,
-	TEST_ACCESS_FLAG,
 };
 
 static u64 *ptep_hva;
@@ -49,7 +48,6 @@ do {											\
 		GUEST_ASSERT_EQ(FIELD_GET(SYS_PAR_EL1_ATTR, par), MAIR_ATTR_NORMAL);	\
 		GUEST_ASSERT_EQ(FIELD_GET(SYS_PAR_EL1_SH, par), PTE_SHARED >> 8);	\
 		GUEST_ASSERT_EQ(par & SYS_PAR_EL1_PA, TEST_ADDR);			\
-		GUEST_SYNC(TEST_ACCESS_FLAG);						\
 	}										\
 } while (0)
 
@@ -85,10 +83,6 @@ static void guest_code(void)
 	if (!SYS_FIELD_GET(ID_AA64MMFR1_EL1, HAFDBS, read_sysreg(id_aa64mmfr1_el1)))
 		GUEST_DONE();
 
-	/*
-	 * KVM's software PTW makes the implementation choice that the AT
-	 * instruction sets the access flag.
-	 */
 	sysreg_clear_set(tcr_el1, 0, TCR_HA);
 	isb();
 	test_at(false);
@@ -102,8 +96,8 @@ static void handle_sync(struct kvm_vcpu *vcpu, struct ucall *uc)
 	case CLEAR_ACCESS_FLAG:
 		/*
 		 * Delete + reinstall the memslot to invalidate stage-2
-		 * mappings of the stage-1 page tables, forcing KVM to
-		 * use the 'slow' AT emulation path.
+		 * mappings of the stage-1 page tables, allowing KVM to
+		 * potentially use the 'slow' AT emulation path.
 		 *
 		 * This and clearing the access flag from host userspace
 		 * ensures that the access flag cannot be set speculatively
@@ -112,10 +106,6 @@ static void handle_sync(struct kvm_vcpu *vcpu, struct ucall *uc)
 		clear_bit(__ffs(PTE_AF), ptep_hva);
 		vm_mem_region_reload(vcpu->vm, vcpu->vm->memslots[MEM_REGION_PT]);
 		break;
-	case TEST_ACCESS_FLAG:
-		TEST_ASSERT(test_bit(__ffs(PTE_AF), ptep_hva),
-			    "Expected access flag to be set (desc: %lu)", *ptep_hva);
-		break;
 	default:
 		TEST_FAIL("Unexpected SYNC arg: %lu", uc->args[1]);
 	}
-- 
2.53.0