[patch 3/8] MDS basics 3

[Thomas Gleixner <tglx@linutronix.de>]

Subject: [patch 3/8] x86/speculation/mds: Clear CPU buffers on exit to user
From: Thomas Gleixner <tglx@linutronix.de>

Add a static key which controls the invocation of the CPU buffer clear
mechanism on exit to user space and add the call into
prepare_exit_to_usermode() right before actually returning.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 arch/x86/entry/common.c              |    3 +++
 arch/x86/include/asm/nospec-branch.h |    2 ++
 arch/x86/kernel/cpu/bugs.c           |    4 +++-
 3 files changed, 8 insertions(+), 1 deletion(-)

--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -31,6 +31,7 @@
 #include <asm/vdso.h>
 #include <linux/uaccess.h>
 #include <asm/cpufeature.h>
+#include <asm/nospec-branch.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
@@ -211,6 +212,8 @@ static void exit_to_usermode_loop(struct
 	ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
 #endif
 
+	mds_clear_cpu_buffers(&user_mds_clear_cpu_buffers);
+
 	user_enter_irqoff();
 }
 
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -337,6 +337,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_
 DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
 DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+DECLARE_STATIC_KEY_FALSE(user_mds_clear_cpu_buffers);
+
 #endif /* __ASSEMBLY__ */
 
 /*
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -63,10 +63,12 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_i
 /* Control unconditional IBPB in switch_mm() */
 DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);
 
+/* Control MDS CPU buffer clear before returning to user space */
+DEFINE_STATIC_KEY_FALSE(user_mds_clear_cpu_buffers);
+
 void __init check_bugs(void)
 {
 	identify_boot_cpu();
-
 	/*
 	 * identify_boot_cpu() initialized SMT support information, let the
 	 * core code know.