x86: Handle idle=nomwait cmdline properly for x86_idle

[ Upstream commit 8bcedb4ce04750e1ccc9a6b6433387f6a9166a56 ]

When kernel is booted with idle=nomwait do not use MWAIT as the
default idle state.

If the user boots the kernel with idle=nomwait, it is a clear
direction to not use mwait as the default idle state.
However, the current code does not take this into consideration
while selecting the default idle state on x86.

Fix it by checking for the idle=nomwait boot option in
prefer_mwait_c1_over_halt().

Also update the documentation around idle=nomwait appropriately.

[ dhansen: tweak commit message ]

Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lkml.kernel.org/r/fdc2dc2d0a1bc21c2f53d989ea2d2ee3ccbc0dbe.1654538381.git-series.wyes.karny@amd.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/x86/kernel/process.c

@@ -777,6 +777,10 @@ static void amd_e400_idle(void)
  */
 static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
 {
+	/* User has disallowed the use of MWAIT. Fallback to HALT */
+	if (boot_option_idle_override == IDLE_NOMWAIT)
+		return 0;
+
 	if (c->x86_vendor != X86_VENDOR_INTEL)
 		return 0;
 
@@ -885,9 +889,8 @@ static int __init idle_setup(char *str)
 	} else if (!strcmp(str, "nomwait")) {
 		/*
 		 * If the boot option of "idle=nomwait" is added,
-		 * it means that mwait will be disabled for CPU C2/C3
-		 * states. In such case it won't touch the variable
-		 * of boot_option_idle_override.
+		 * it means that mwait will be disabled for CPU C1/C2/C3
+		 * states.
 		 */
 		boot_option_idle_override = IDLE_NOMWAIT;
 	} else