Re: [RFC PATCH v2 32/32] x86/fred: disable FRED by default in its early stage

["H. Peter Anvin" <hpa@zytor.com>]

On January 6, 2023 12:56:17 AM PST, Xin Li <xin3.li@intel.com> wrote:
>Disable FRED by default in its early stage.
>
>To enable FRED, a new kernel command line option "fred" needs to be added.
>
>Signed-off-by: Xin Li <xin3.li@intel.com>
>---
> Documentation/admin-guide/kernel-parameters.txt | 4 ++++
> arch/x86/kernel/cpu/common.c                    | 3 +++
> 2 files changed, 7 insertions(+)
>
>diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>index 42af9ca0127e..0bc76d926dd4 100644
>--- a/Documentation/admin-guide/kernel-parameters.txt
>+++ b/Documentation/admin-guide/kernel-parameters.txt
>@@ -1506,6 +1506,10 @@
> 			Warning: use of this parameter will taint the kernel
> 			and may cause unknown problems.
> 
>+	fred
>+			Forcefully enable flexible return and event delivery,
>+			which is otherwise disabled by default.
>+
> 	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/cpu/common.c b/arch/x86/kernel/cpu/common.c
>index 5de68356fe62..1a160337ad41 100644
>--- a/arch/x86/kernel/cpu/common.c
>+++ b/arch/x86/kernel/cpu/common.c
>@@ -1434,6 +1434,9 @@ static void __init cpu_parse_early_param(void)
> 	char *argptr = arg, *opt;
> 	int arglen, taint = 0;
> 
>+	if (!cmdline_find_option_bool(boot_command_line, "fred"))
>+		setup_clear_cpu_cap(X86_FEATURE_FRED);
>+
> #ifdef CONFIG_X86_32
> 	if (cmdline_find_option_bool(boot_command_line, "no387"))
> #ifdef CONFIG_MATH_EMULATION

Specifically, the intent is to enable it by default once hardware is widely available. This allows e.g. distros to compile in the support for opt-in field testing before throwing the switch.

The motivation for this comes from the SMAP experience where the code was functional and correct when merged, but a change elsewhere in the kernel code base broke it during the intervening period.