[MODERATED] [PATCH v4 02/28] MDSv4 22

[Andi Kleen <andi@firstfloor.org>]

Normally we execute VERW for clearing the cpu unconditionally on kernel exits
that might have touched sensitive. Add a new flag to disable VERW usage.
This is intended for systems that only run trusted code and don't
want the performance impact of the extra clearing.

This just sets the flag, actual implementation is in future patches.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 arch/x86/include/asm/cpufeatures.h              | 1 +
 arch/x86/kernel/cpu/bugs.c                      | 9 +++++++++
 3 files changed, 13 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b799bcf67d7b..9c967d0caeca 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2357,6 +2357,9 @@
 			Format: <first>,<last>
 			Specifies range of consoles to be captured by the MDA.
 
+	mds=off		[X86, Intel]
+			Disable workarounds for Micro-architectural Data Sampling.
+
 	mem=nn[KMG]	[KNL,BOOT] Force usage of a specific amount of memory
 			Amount of memory to be used when the kernel is not able
 			to see the whole system memory or for test.
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 233ca598826f..09347c6a8901 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -221,6 +221,7 @@
 #define X86_FEATURE_ZEN			( 7*32+28) /* "" CPU is AMD family 0x17 (Zen) */
 #define X86_FEATURE_L1TF_PTEINV		( 7*32+29) /* "" L1TF workaround PTE inversion */
 #define X86_FEATURE_IBRS_ENHANCED	( 7*32+30) /* Enhanced IBRS */
+#define X86_FEATURE_NO_VERW		( 7*32+31) /* "" No VERW for MDS on kernel exit */
 
 /* Virtualization flags: Linux defined, word 8 */
 #define X86_FEATURE_TPR_SHADOW		( 8*32+ 0) /* Intel TPR Shadow */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 8654b8b0c848..5426467143c9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -37,6 +37,7 @@
 static void __init spectre_v2_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
+static void __init mds_select_mitigation(void);
 
 /* The base value of the SPEC_CTRL MSR that always has to be preserved. */
 u64 x86_spec_ctrl_base;
@@ -101,6 +102,8 @@ void __init check_bugs(void)
 
 	l1tf_select_mitigation();
 
+	mds_select_mitigation();
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Check whether we are able to run this kernel safely on SMP.
@@ -1058,6 +1061,12 @@ early_param("l1tf", l1tf_cmdline);
 
 #undef pr_fmt
 
+static void mds_select_mitigation(void)
+{
+	if (cmdline_find_option_bool(boot_command_line, "mds=off"))
+		setup_force_cpu_cap(X86_FEATURE_NO_VERW);
+}
+
 #ifdef CONFIG_SYSFS
 
 #define L1TF_DEFAULT_MSG "Mitigation: PTE Inversion"
-- 
2.17.2