KVM: arm64: selftests: Disable single-step without relying on ucall()

Automatically disable single-step when the guest reaches the end of the
verified section instead of using an explicit ucall() to ask userspace to
disable single-step.  An upcoming change to implement a pool-based scheme
for ucall() will add an atomic operation (bit test and set) in the guest
ucall code, and if the compiler generate "old school" atomics, e.g.

  40e57c:       c85f7c20        ldxr    x0, [x1]
  40e580:       aa100011        orr     x17, x0, x16
  40e584:       c80ffc31        stlxr   w15, x17, [x1]
  40e588:       35ffffaf        cbnz    w15, 40e57c <__aarch64_ldset8_sync+0x1c>

the guest will hang as the local exclusive monitor is reset by eret,
i.e. the stlxr will always fail due to the debug exception taken to EL2.

Link: https://lore.kernel.org/all/20221006003409.649993-8-seanjc@google.com
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221117002350.2178351-3-seanjc@google.comReviewed-by: Oliver Upton <oliver.upton@linux.dev>

tools/testing/selftests/kvm/aarch64/debug-exceptions.c

@@ -241,7 +241,6 @@ static void guest_svc_handler(struct ex_regs *regs)
 
 enum single_step_op {
 	SINGLE_STEP_ENABLE = 0,
-	SINGLE_STEP_DISABLE = 1,
 };
 
 static void guest_code_ss(int test_cnt)
@@ -258,7 +257,7 @@ static void guest_code_ss(int test_cnt)
 		GUEST_SYNC(SINGLE_STEP_ENABLE);
 
 		/*
-		 * The userspace will veriry that the pc is as expected during
+		 * The userspace will verify that the pc is as expected during
 		 * single step execution between iter_ss_begin and iter_ss_end.
 		 */
 		asm volatile("iter_ss_begin:nop\n");
@@ -268,11 +267,9 @@ static void guest_code_ss(int test_cnt)
 		bvr = read_sysreg(dbgbvr0_el1);
 		wvr = read_sysreg(dbgwvr0_el1);
 
+		/* Userspace disables Single Step when the end is nigh. */
 		asm volatile("iter_ss_end:\n");
 
-		/* Disable Single Step execution */
-		GUEST_SYNC(SINGLE_STEP_DISABLE);
-
 		GUEST_ASSERT(bvr == w_bvr);
 		GUEST_ASSERT(wvr == w_wvr);
 	}
@@ -364,15 +361,12 @@ void test_single_step_from_userspace(int test_cnt)
 			TEST_ASSERT(cmd == UCALL_SYNC,
 				    "Unexpected ucall cmd 0x%lx", cmd);
 
-			if (uc.args[1] == SINGLE_STEP_ENABLE) {
-				debug.control = KVM_GUESTDBG_ENABLE |
-						KVM_GUESTDBG_SINGLESTEP;
-				ss_enable = true;
-			} else {
-				debug.control = KVM_GUESTDBG_ENABLE;
-				ss_enable = false;
-			}
+			TEST_ASSERT(uc.args[1] == SINGLE_STEP_ENABLE,
+				    "Unexpected ucall action 0x%lx", uc.args[1]);
 
+			debug.control = KVM_GUESTDBG_ENABLE |
+					KVM_GUESTDBG_SINGLESTEP;
+			ss_enable = true;
 			vcpu_guest_debug_set(vcpu, &debug);
 			continue;
 		}
@@ -385,6 +379,14 @@ void test_single_step_from_userspace(int test_cnt)
 			    "Unexpected pc 0x%lx (expected 0x%lx)",
 			    pc, test_pc);
 
+		if ((pc + 4) == (uint64_t)&iter_ss_end) {
+			test_pc = 0;
+			debug.control = KVM_GUESTDBG_ENABLE;
+			ss_enable = false;
+			vcpu_guest_debug_set(vcpu, &debug);
+			continue;
+		}
+
 		/*
 		 * If the current pc is between iter_ss_bgin and
 		 * iter_ss_end, the pc for the next KVM_EXIT_DEBUG should