[PATCH v4 01/11] x86/bugs: Add a separate config for MDS

[Breno Leitao]

Currently, the CONFIG_SPECULATION_MITIGATIONS is halfway populated,
where some mitigations have entries in Kconfig, and they could be
modified, while others mitigations do not have Kconfig entries, and
could not be controlled at build time.

Create an entry for the MDS CPU mitigation under
CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
it at compilation time.

Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
 arch/x86/Kconfig           | 9 +++++++++
 arch/x86/kernel/cpu/bugs.c | 3 ++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 007bab9f2a0e..36e871ab1ef9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2650,6 +2650,15 @@ config MITIGATION_SPECTRE_BHI
 	  indirect branches.
 	  See <file:Documentation/admin-guide/hw-vuln/spectre.rst>
 
+config MITIGATION_MDS
+	bool "Mitigate Microarchitectural Data Sampling (MDS) hardware bug"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Microarchitectural Data Sampling (MDS). MDS is
+	  a hardware vulnerability which allows unprivileged speculative access
+	  to data which is available in various CPU internal buffers.
+	  See also <file:Documentation/admin-guide/hw-vuln/mds.rst>
 endif
 
 config ARCH_HAS_ADD_PAGES
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 45675da354f3..dbfc7d5c5f48 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -233,7 +233,8 @@ static void x86_amd_ssb_disable(void)
 #define pr_fmt(fmt)	"MDS: " fmt
 
 /* Default mitigation for MDS-affected CPUs */
-static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
+static enum mds_mitigations mds_mitigation __ro_after_init =
+	IS_ENABLED(CONFIG_MITIGATION_MDS) ? MDS_MITIGATION_FULL : MDS_MITIGATION_OFF;
 static bool mds_nosmt __ro_after_init = false;
 
 static const char * const mds_strings[] = {
-- 
2.43.0