x86/tsx: Add "auto" option to the tsx= cmdline parameter

Platforms which are not affected by X86_BUG_TAA may want the TSX feature
enabled. Add "auto" option to the TSX cmdline parameter. When tsx=auto
disable TSX when X86_BUG_TAA is present, otherwise enable TSX.

More details on X86_BUG_TAA can be found here:
https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/tsx_async_abort.html

 [ bp: Extend the arg buffer to accommodate "auto\0". ]
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -4869,6 +4869,9 @@
 				update. This new MSR allows for the reliable
 				deactivation of the TSX functionality.)
 
+			auto	- Disable TSX if X86_BUG_TAA is present,
+				  otherwise enable TSX on the system.
+
 			Not specifying this option is equivalent to tsx=off.
 
 			See Documentation/admin-guide/hw-vuln/tsx_async_abort.rst

arch/x86/kernel/cpu/tsx.c

@@ -75,7 +75,7 @@ static bool __init tsx_ctrl_is_supported(void)
 
 void __init tsx_init(void)
 {
-	char arg[4] = {};
+	char arg[5] = {};
 	int ret;
 
 	if (!tsx_ctrl_is_supported())
@@ -87,6 +87,11 @@ void __init tsx_init(void)
 			tsx_ctrl_state = TSX_CTRL_ENABLE;
 		} else if (!strcmp(arg, "off")) {
 			tsx_ctrl_state = TSX_CTRL_DISABLE;
+		} else if (!strcmp(arg, "auto")) {
+			if (boot_cpu_has_bug(X86_BUG_TAA))
+				tsx_ctrl_state = TSX_CTRL_DISABLE;
+			else
+				tsx_ctrl_state = TSX_CTRL_ENABLE;
 		} else {
 			tsx_ctrl_state = TSX_CTRL_DISABLE;
 			pr_err("tsx: invalid option, defaulting to off\n");