linux-kernel - RE: [RFC PATCH v2 32/32] x86/fred: disable FRED by default in its early stage

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Message-ID: <SA1PR11MB673442AED398A6BDCD50C700A8FB9@SA1PR11MB6734.namprd11.prod.outlook.com>
Date:   Fri, 6 Jan 2023 23:00:14 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "H. Peter Anvin" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [RFC PATCH v2 32/32] x86/fred: disable FRED by default in its
 early stage

> >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@...el.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.

Thanks for the clarification.