tests/tcg/x86_64: Add cross-modifying code test

commit f025692c99 ("accel/tcg: Clear PAGE_WRITE before translation")
fixed cross-modifying code handling, but did not add a test. The
changed code was further improved recently [1], and I was not sure
whether these modifications were safe (spoiler: they were fine).

Add a test to make sure there are no regressions.

[1] https://lists.gnu.org/archive/html/qemu-devel/2022-09/msg00034.html

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20241001150617.9977-1-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241023113406.1284676-8-alex.bennee@linaro.org>

tests/tcg/x86_64/Makefile.target

@@ -17,6 +17,7 @@ X86_64_TESTS += cmpxchg
 X86_64_TESTS += adox
 X86_64_TESTS += test-1648
 X86_64_TESTS += test-2175
+X86_64_TESTS += cross-modifying-code
 TESTS=$(MULTIARCH_TESTS) $(X86_64_TESTS) test-x86_64
 else
 TESTS=$(MULTIARCH_TESTS)
@@ -27,6 +28,9 @@ adox: CFLAGS=-O2
 run-test-i386-ssse3: QEMU_OPTS += -cpu max
 run-plugin-test-i386-ssse3-%: QEMU_OPTS += -cpu max
 
+cross-modifying-code: CFLAGS+=-pthread
+cross-modifying-code: LDFLAGS+=-pthread
+
 test-x86_64: LDFLAGS+=-lm -lc
 test-x86_64: test-i386.c test-i386.h test-i386-shift.h test-i386-muldiv.h
 	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)

tests/tcg/x86_64/cross-modifying-code.c

@@ -0,0 +1,80 @@
+/*
+ * Test patching code, running in one thread, from another thread.
+ *
+ * Intel SDM calls this "cross-modifying code" and recommends a special
+ * sequence, which requires both threads to cooperate.
+ *
+ * Linux kernel uses a different sequence that does not require cooperation and
+ * involves patching the first byte with int3.
+ *
+ * Finally, there is user-mode software out there that simply uses atomics, and
+ * that seems to be good enough in practice. Test that QEMU has no problems
+ * with this as well.
+ */
+
+#include <assert.h>
+#include <pthread.h>
+#include <stdbool.h>
+#include <stdlib.h>
+
+void add1_or_nop(long *x);
+asm(".pushsection .rwx,\"awx\",@progbits\n"
+    ".globl add1_or_nop\n"
+    /* addq $0x1,(%rdi) */
+    "add1_or_nop: .byte 0x48, 0x83, 0x07, 0x01\n"
+    "ret\n"
+    ".popsection\n");
+
+#define THREAD_WAIT 0
+#define THREAD_PATCH 1
+#define THREAD_STOP 2
+
+static void *thread_func(void *arg)
+{
+    int val = 0x0026748d; /* nop */
+
+    while (true) {
+        switch (__atomic_load_n((int *)arg, __ATOMIC_SEQ_CST)) {
+        case THREAD_WAIT:
+            break;
+        case THREAD_PATCH:
+            val = __atomic_exchange_n((int *)&add1_or_nop, val,
+                                      __ATOMIC_SEQ_CST);
+            break;
+        case THREAD_STOP:
+            return NULL;
+        default:
+            assert(false);
+            __builtin_unreachable();
+        }
+    }
+}
+
+#define INITIAL 42
+#define COUNT 1000000
+
+int main(void)
+{
+    int command = THREAD_WAIT;
+    pthread_t thread;
+    long x = 0;
+    int err;
+    int i;
+
+    err = pthread_create(&thread, NULL, &thread_func, &command);
+    assert(err == 0);
+
+    __atomic_store_n(&command, THREAD_PATCH, __ATOMIC_SEQ_CST);
+    for (i = 0; i < COUNT; i++) {
+        add1_or_nop(&x);
+    }
+    __atomic_store_n(&command, THREAD_STOP, __ATOMIC_SEQ_CST);
+
+    err = pthread_join(thread, NULL);
+    assert(err == 0);
+
+    assert(x >= INITIAL);
+    assert(x <= INITIAL + COUNT);
+
+    return EXIT_SUCCESS;
+}