x86/fred: Add a fred= cmdline param

Let command line option "fred" accept multiple options to make it
easier to tweak its behavior.

Currently, two options 'on' and 'off' are allowed, and the default
behavior is to disable FRED. To enable FRED, append "fred=on" to the
kernel command line.

  [ bp: Use cpu_feature_enabled(), touch ups. ]

Signed-off-by: Xin Li <xin3.li@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20231205105030.8698-9-xin3.li@intel.com

2 files changed, 32 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 31b3a25680d0..d6ea4f42127c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1539,6 +1539,12 @@
 			Warning: use of this parameter will taint the kernel
 			and may cause unknown problems.
 
+	fred=		[X86-64]
+			Enable/disable Flexible Return and Event Delivery.
+			Format: { on | off }
+			on: enable FRED when it's present.
+			off: disable FRED, the default setting.
+
 	ftrace=[tracer]
 			[FTRACE] will set and start the specified tracer
 			as early as possible in order to facilitate early
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c3b2f863acf0..3c3748901825 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1369,8 +1369,34 @@ DEFINE_IDTENTRY_SW(iret_error)
 }
 #endif
 
+/* Do not enable FRED by default yet. */
+static bool enable_fred __ro_after_init = false;
+
+#ifdef CONFIG_X86_FRED
+static int __init fred_setup(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!cpu_feature_enabled(X86_FEATURE_FRED))
+		return 0;
+
+	if (!strcmp(str, "on"))
+		enable_fred = true;
+	else if (!strcmp(str, "off"))
+		enable_fred = false;
+	else
+		pr_warn("invalid FRED option: 'fred=%s'\n", str);
+	return 0;
+}
+early_param("fred", fred_setup);
+#endif
+
 void __init trap_init(void)
 {
+	if (cpu_feature_enabled(X86_FEATURE_FRED) && !enable_fred)
+		setup_clear_cpu_cap(X86_FEATURE_FRED);
+
 	/* Init cpu_entry_area before IST entries are set up */
 	setup_cpu_entry_areas();