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.com
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>

diff --git a/tools/testing/selftests/kvm/aarch64/debug-exceptions.c b/tools/testing/selftests/kvm/aarch64/debug-exceptions.c
index 0316f22..a3c2216 100644 (file)
--- a/tools/testing/selftests/kvm/aarch64/debug-exceptions.c
+++ b/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