x86/bugs: Remove CONFIG_BHI_MITIGATION_AUTO and spectre_bhi=auto

Unlike most other mitigations' "auto" options, spectre_bhi=auto only
mitigates newer systems, which is confusing and not particularly useful.

Remove it.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/412e9dc87971b622bbbaf64740ebc1f140bff343.1712813475.git.jpoimboe@kernel.org

Author: jpoimboe

Documentation/admin-guide/hw-vuln/spectre.rst

@@ -669,10 +669,6 @@ kernel command line.
 			needed.
 		off
 			Disable the mitigation.
-		auto
-			Enable the HW mitigation if needed, but
-			*don't* enable the SW mitigation except for KVM.
-			The system may be vulnerable.
 
 For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
 

Documentation/admin-guide/kernel-parameters.txt

@@ -6072,9 +6072,6 @@
 			on   - (default) Enable the HW or SW mitigation
 			       as needed.
 			off  - Disable the mitigation.
-			auto - Enable the HW mitigation if needed, but
-			       *don't* enable the SW mitigation except
-			       for KVM.  The system may be vulnerable.
 
 	spectre_v2=	[X86,EARLY] Control mitigation of Spectre variant 2
 			(indirect branch speculation) vulnerability.

arch/x86/Kconfig

@@ -2651,11 +2651,6 @@ config SPECTRE_BHI_OFF
 	bool "off"
 	help
 	  Equivalent to setting spectre_bhi=off command line parameter.
-config SPECTRE_BHI_AUTO
-	bool "auto"
-	depends on BROKEN
-	help
-	  Equivalent to setting spectre_bhi=auto command line parameter.
 
 endchoice
 

arch/x86/kernel/cpu/bugs.c

@@ -1625,13 +1625,10 @@ static bool __init spec_ctrl_bhi_dis(void)
 enum bhi_mitigations {
 	BHI_MITIGATION_OFF,
 	BHI_MITIGATION_ON,
-	BHI_MITIGATION_AUTO,
 };
 
 static enum bhi_mitigations bhi_mitigation __ro_after_init =
-	IS_ENABLED(CONFIG_SPECTRE_BHI_ON)  ? BHI_MITIGATION_ON  :
-	IS_ENABLED(CONFIG_SPECTRE_BHI_OFF) ? BHI_MITIGATION_OFF :
-					     BHI_MITIGATION_AUTO;
+	IS_ENABLED(CONFIG_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
 
 static int __init spectre_bhi_parse_cmdline(char *str)
 {
@@ -1642,8 +1639,6 @@ static int __init spectre_bhi_parse_cmdline(char *str)
 		bhi_mitigation = BHI_MITIGATION_OFF;
 	else if (!strcmp(str, "on"))
 		bhi_mitigation = BHI_MITIGATION_ON;
-	else if (!strcmp(str, "auto"))
-		bhi_mitigation = BHI_MITIGATION_AUTO;
 	else
 		pr_err("Ignoring unknown spectre_bhi option (%s)", str);
 
@@ -1673,9 +1668,6 @@ static void __init bhi_select_mitigation(void)
 	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
 	pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
 
-	if (bhi_mitigation == BHI_MITIGATION_AUTO)
-		return;
-
 	/* Mitigate syscalls when the mitigation is forced =on */
 	setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
 	pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");