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Subject[PATCH v8 15/17] x86/fsgsbase/64: Enable FSGSBASE on 64bit by default and add a chicken bit
Date
From: Andy Lutomirski <luto@kernel.org>

Now that FSGSBASE is fully supported, remove unsafe_fsgsbase, enable
FSGSBASE by default, and add nofsgsbase to disable it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
---

Changes from v7:
* No code change
* Massaged title by Thomas
---
Documentation/admin-guide/kernel-parameters.txt | 3 +--
arch/x86/kernel/cpu/common.c | 32 +++++++++++--------------
2 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2ff72af..ce11a6a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2875,8 +2875,7 @@
no5lvl [X86-64] Disable 5-level paging mode. Forces
kernel to use 4-level paging instead.

- unsafe_fsgsbase [X86] Allow FSGSBASE instructions. This will be
- replaced with a nofsgsbase flag.
+ nofsgsbase [X86] Disables FSGSBASE instructions.

no_console_suspend
[HW] Never suspend the console
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 9f57fb0..0fe75fa 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -437,21 +437,21 @@ static void __init setup_cr_pinning(void)
static_key_enable(&cr_pinning.key);
}

-/*
- * Temporary hack: FSGSBASE is unsafe until a few kernel code paths are
- * updated. This allows us to get the kernel ready incrementally.
- *
- * Once all the pieces are in place, these will go away and be replaced with
- * a nofsgsbase chicken flag.
- */
-static bool unsafe_fsgsbase;
-
-static __init int setup_unsafe_fsgsbase(char *arg)
+static __init int x86_nofsgsbase_setup(char *arg)
{
- unsafe_fsgsbase = true;
+ /* Require an exact match without trailing characters. */
+ if (strlen(arg))
+ return 0;
+
+ /* Do not emit a message if the feature is not present. */
+ if (!boot_cpu_has(X86_FEATURE_FSGSBASE))
+ return 1;
+
+ setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
+ pr_info("nofsgsbase: FSGSBASE disabled\n");
return 1;
}
-__setup("unsafe_fsgsbase", setup_unsafe_fsgsbase);
+__setup("nofsgsbase", x86_nofsgsbase_setup);

/*
* Protection Keys are not available in 32-bit mode.
@@ -1472,12 +1472,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
setup_umip(c);

/* Enable FSGSBASE instructions if available. */
- if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
- if (unsafe_fsgsbase)
- cr4_set_bits(X86_CR4_FSGSBASE);
- else
- clear_cpu_cap(c, X86_FEATURE_FSGSBASE);
- }
+ if (cpu_has(c, X86_FEATURE_FSGSBASE))
+ cr4_set_bits(X86_CR4_FSGSBASE);

/*
* The vendor-specific functions might have changed features.
--
2.7.4
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