[patch 16/24] x86/speculation: Prepare for per task indirect branch speculation control

[Thomas Gleixner]

From: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

To avoid the overhead of STIBP always on, it's necessary to allow per task
control of STIBP.

Add a new task flag TIF_SPEC_IB and evaluate it during context switch if
SMT is active and flag evaluation is enabled by the speculation control
code. Add the conditional evaluation to x86_virt_spec_ctrl() as well so the
guest/host switch works properly.

This has no effect because TIF_SPEC_IB cannot be set yet and the static key
which controls evaluation is off. Preparatory patch for adding the control
code.

[ tglx: Simplify the context switch logic and make the TIF evaluation
  	depend on SMP=y and on the static key controlling the conditional
  	update. Rename it to TIF_SPEC_IB because it controls both STIBP and
  	IBPB ]

Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

---
 arch/x86/include/asm/msr-index.h   |    5 +++--
 arch/x86/include/asm/spec-ctrl.h   |   12 ++++++++++++
 arch/x86/include/asm/thread_info.h |    5 ++++-
 arch/x86/kernel/cpu/bugs.c         |    4 ++++
 arch/x86/kernel/process.c          |   24 ++++++++++++++++++++++--
 5 files changed, 45 insertions(+), 5 deletions(-)

--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -41,9 +41,10 @@
 
 #define MSR_IA32_SPEC_CTRL		0x00000048 /* Speculation Control */
 #define SPEC_CTRL_IBRS			(1 << 0)   /* Indirect Branch Restricted Speculation */
-#define SPEC_CTRL_STIBP			(1 << 1)   /* Single Thread Indirect Branch Predictors */
+#define SPEC_CTRL_STIBP_SHIFT		1	   /* Single Thread Indirect Branch Predictor (STIBP) bit */
+#define SPEC_CTRL_STIBP			(1 << SPEC_CTRL_STIBP_SHIFT)	/* STIBP mask */
 #define SPEC_CTRL_SSBD_SHIFT		2	   /* Speculative Store Bypass Disable bit */
-#define SPEC_CTRL_SSBD			(1 << SPEC_CTRL_SSBD_SHIFT)   /* Speculative Store Bypass Disable */
+#define SPEC_CTRL_SSBD			(1 << SPEC_CTRL_SSBD_SHIFT)	/* Speculative Store Bypass Disable */
 
 #define MSR_IA32_PRED_CMD		0x00000049 /* Prediction Command */
 #define PRED_CMD_IBPB			(1 << 0)   /* Indirect Branch Prediction Barrier */
--- a/arch/x86/include/asm/spec-ctrl.h
+++ b/arch/x86/include/asm/spec-ctrl.h
@@ -53,12 +53,24 @@ static inline u64 ssbd_tif_to_spec_ctrl(
 	return (tifn & _TIF_SSBD) >> (TIF_SSBD - SPEC_CTRL_SSBD_SHIFT);
 }
 
+static inline u64 stibp_tif_to_spec_ctrl(u64 tifn)
+{
+	BUILD_BUG_ON(TIF_SPEC_IB < SPEC_CTRL_STIBP_SHIFT);
+	return (tifn & _TIF_SPEC_IB) >> (TIF_SPEC_IB - SPEC_CTRL_STIBP_SHIFT);
+}
+
 static inline unsigned long ssbd_spec_ctrl_to_tif(u64 spec_ctrl)
 {
 	BUILD_BUG_ON(TIF_SSBD < SPEC_CTRL_SSBD_SHIFT);
 	return (spec_ctrl & SPEC_CTRL_SSBD) << (TIF_SSBD - SPEC_CTRL_SSBD_SHIFT);
 }
 
+static inline unsigned long stibp_spec_ctrl_to_tif(u64 spec_ctrl)
+{
+	BUILD_BUG_ON(TIF_SPEC_IB < SPEC_CTRL_STIBP_SHIFT);
+	return (spec_ctrl & SPEC_CTRL_STIBP) << (TIF_SPEC_IB - SPEC_CTRL_STIBP_SHIFT);
+}
+
 static inline u64 ssbd_tif_to_amd_ls_cfg(u64 tifn)
 {
 	return (tifn & _TIF_SSBD) ? x86_amd_ls_cfg_ssbd_mask : 0ULL;
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -83,6 +83,7 @@ struct thread_info {
 #define TIF_SYSCALL_EMU		6	/* syscall emulation active */
 #define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
 #define TIF_SECCOMP		8	/* secure computing */
+#define TIF_SPEC_IB		9	/* Indirect branch speculation mitigation */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
 #define TIF_UPROBE		12	/* breakpointed or singlestepping */
 #define TIF_PATCH_PENDING	13	/* pending live patching update */
@@ -110,6 +111,7 @@ struct thread_info {
 #define _TIF_SYSCALL_EMU	(1 << TIF_SYSCALL_EMU)
 #define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
 #define _TIF_SECCOMP		(1 << TIF_SECCOMP)
+#define _TIF_SPEC_IB		(1 << TIF_SPEC_IB)
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
 #define _TIF_UPROBE		(1 << TIF_UPROBE)
 #define _TIF_PATCH_PENDING	(1 << TIF_PATCH_PENDING)
@@ -146,7 +148,8 @@ struct thread_info {
 
 /* flags to check in __switch_to() */
 #define _TIF_WORK_CTXSW							\
-	(_TIF_IO_BITMAP|_TIF_NOCPUID|_TIF_NOTSC|_TIF_BLOCKSTEP|_TIF_SSBD)
+	(_TIF_IO_BITMAP|_TIF_NOCPUID|_TIF_NOTSC|_TIF_BLOCKSTEP|		\
+	 _TIF_SSBD|_TIF_SPEC_IB)
 
 #define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
 #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -148,6 +148,10 @@ x86_virt_spec_ctrl(u64 guest_spec_ctrl,
 		    static_cpu_has(X86_FEATURE_AMD_SSBD))
 			hostval |= ssbd_tif_to_spec_ctrl(ti->flags);
 
+		/* Check whether dynamic indirect branch control is on */
+		if (static_branch_unlikely(&switch_to_cond_stibp))
+			hostval |= stibp_tif_to_spec_ctrl(ti->flags);
+
 		if (hostval != guestval) {
 			msrval = setguest ? guestval : hostval;
 			wrmsrl(MSR_IA32_SPEC_CTRL, msrval);
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -12,6 +12,7 @@
 #include <linux/sched/debug.h>
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
+#include <linux/sched/topology.h>
 #include <linux/init.h>
 #include <linux/export.h>
 #include <linux/pm.h>
@@ -406,6 +407,11 @@ static __always_inline void spec_ctrl_up
 	if (static_cpu_has(X86_FEATURE_SSBD))
 		msr |= ssbd_tif_to_spec_ctrl(tifn);
 
+	/* Only evaluate STIBP if dynamic control is enabled */
+	if (IS_ENABLED(CONFIG_SMP) &&
+	    static_branch_unlikely(&switch_to_cond_stibp))
+		msr |= stibp_tif_to_spec_ctrl(tifn);
+
 	wrmsrl(MSR_IA32_SPEC_CTRL, msr);
 }
 
@@ -418,10 +424,16 @@ static __always_inline void spec_ctrl_up
 static __always_inline void __speculation_ctrl_update(unsigned long tifp,
 						      unsigned long tifn)
 {
+	unsigned long tif_diff = tifp ^ tifn;
 	bool updmsr = false;
 
-	/* If TIF_SSBD is different, select the proper mitigation method */
-	if ((tifp ^ tifn) & _TIF_SSBD) {
+	/*
+	 * If TIF_SSBD is different, select the proper mitigation
+	 * method. Note that if SSBD mitigation is disabled or permanentely
+	 * enabled this branch can't be taken because nothing can set
+	 * TIF_SSBD.
+	 */
+	if (tif_diff & _TIF_SSBD) {
 		if (static_cpu_has(X86_FEATURE_VIRT_SSBD))
 			amd_set_ssb_virt_state(tifn);
 		else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
@@ -430,6 +442,14 @@ static __always_inline void __speculatio
 			updmsr  = true;
 	}
 
+	/*
+	 * Only evaluate TIF_SPEC_IB if dynamic control is
+	 * enabled, otherwise avoid the MSR write
+	 */
+	if (IS_ENABLED(CONFIG_SMP) &&
+	    static_branch_unlikely(&switch_to_cond_stibp))
+		updmsr |= !!(tif_diff & _TIF_SPEC_IB);
+
 	if (updmsr)
 		spec_ctrl_update_msr(tifn);
 }