[patch V2 07/28] x86/speculation: Reorganize speculation control MSRs update

[Thomas Gleixner]

The logic to detect whether there's a change in the previous and next
task's flag relevant to update speculation control MSRs are spread out
across multiple functions.

Consolidate all checks needed for updating speculation control MSRs into
the new __speculation_ctrl_update() helper function.

This makes it easy to pick the right speculation control MSR and the bits
in the MSR that needs updating based on TIF flags changes.

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

---
 arch/x86/kernel/process.c |   42 ++++++++++++++++++++++++++++++++----------
 1 file changed, 32 insertions(+), 10 deletions(-)

--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -397,25 +397,48 @@ static __always_inline void amd_set_ssb_
 
 static __always_inline void spec_ctrl_update_msr(unsigned long tifn)
 {
-	u64 msr = x86_spec_ctrl_base | ssbd_tif_to_spec_ctrl(tifn);
+	u64 msr = x86_spec_ctrl_base;
+
+	/*
+	 * If X86_FEATURE_SSBD is not set, the SSBD bit is not to be
+	 * touched.
+	 */
+	if (static_cpu_has(X86_FEATURE_SSBD))
+		msr |= ssbd_tif_to_spec_ctrl(tifn);
 
 	wrmsrl(MSR_IA32_SPEC_CTRL, msr);
 }
 
-static __always_inline void __speculation_ctrl_update(unsigned long tifn)
+/*
+ * Update the MSRs managing speculation control, during context switch.
+ *
+ * tifp: Previous task's thread flags
+ * tifn: Next task's thread flags
+ */
+static __always_inline void __speculation_ctrl_update(unsigned long tifp,
+						      unsigned long tifn)
 {
-	if (static_cpu_has(X86_FEATURE_VIRT_SSBD))
-		amd_set_ssb_virt_state(tifn);
-	else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD))
-		amd_set_core_ssb_state(tifn);
-	else
+	bool updmsr = false;
+
+	/* If TIF_SSBD is different, select the proper mitigation method */
+	if ((tifp ^ tifn) & _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))
+			amd_set_core_ssb_state(tifn);
+		else if (static_cpu_has(X86_FEATURE_SSBD))
+			updmsr  = true;
+	}
+
+	if (updmsr)
 		spec_ctrl_update_msr(tifn);
 }
 
 void speculation_ctrl_update(unsigned long tif)
 {
+	/* Forced update. Make sure all relevant TIF flags are different */
 	preempt_disable();
-	__speculation_ctrl_update(tif);
+	__speculation_ctrl_update(~tif, tif);
 	preempt_enable();
 }
 
@@ -451,8 +474,7 @@ void __switch_to_xtra(struct task_struct
 	if ((tifp ^ tifn) & _TIF_NOCPUID)
 		set_cpuid_faulting(!!(tifn & _TIF_NOCPUID));
 
-	if ((tifp ^ tifn) & _TIF_SSBD)
-		__speculation_ctrl_update(tifn);
+	__speculation_ctrl_update(tifp, tifn);
 }
 
 /*