[PATCH 5/5] x86: Support reverse execution

[Frederic Weisbecker <fweisbec@gmail.com>]

Support reverse execution for negative number of CPUs. We might be able
to implement that deeper with the function tracer.

No-Yet-Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 arch/x86/kernel/cpu/negative.c | 28 ++++++++++++++++++++++++++++
 arch/x86/kernel/head64.c       |  4 ++++
 2 files changed, 32 insertions(+)
 create mode 100644 arch/x86/kernel/cpu/negative.c

diff --git a/arch/x86/kernel/cpu/negative.c b/arch/x86/kernel/cpu/negative.c
new file mode 100644
index 0000000..3aff473
--- /dev/null
+++ b/arch/x86/kernel/cpu/negative.c
@@ -0,0 +1,28 @@
+/* Execute function in reverse for negative CPUs */
+
+#define _ASM_RET 0xc3
+#define FUNC_MAX_SIZE 4096
+static DEFINE_PER_CPU(char, reverse_func_buf[FUNC_MAX_SIZE]);
+
+void execute_reverse_function(void *func)
+{
+	char *opcode = (char *)func;
+	int i = 0, j = FUNC_MAX_SIZE - 1;
+	void (*reverse_func)(void);
+
+	/*
+	 * Reverse copy the function.
+	 * This assumes that every opcode is sizeof(char),
+	 * there might be a few situations where that won't work
+	 * but we'll fix them as soon as they get reported by users.
+	 */
+	while (1) {
+		reverse_func_buf[j--] = opcode[i];
+		if (opcode[i++] == _ASM_RET)
+			break;
+	}
+
+	reverse_func = &reverse_func_buf[FUNC_MAX_SIZE - 1];
+	reverse_func();
+}
+
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index c4f8d46..9ed2c88 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -197,5 +197,9 @@ void __init x86_64_start_reservations(char *real_mode_data)
 
 	reserve_ebda_region();
 
+#if CONFIG_NR_CPUS >= 0
 	start_kernel();
+#else
+	execute_reverse_function(start_kernel);
+#endif
 }
-- 
2.1.4