[PATCH v2 2/5] x86/idle: Disable IBRS when cpu is offline

[Waiman Long]

Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
disables IBRS when the CPU enters long idle. However, when a CPU becomes
offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS is
enabled. That will impact the performance of a sibling CPU. Mitigate
this performance impact by clearing all the mitigation bits in SPEC_CTRL
MSR when offline and restoring the value of the MSR when it becomes
online again.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 arch/x86/kernel/smpboot.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 352f0ce1ece4..5ff82fef413c 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -84,6 +84,7 @@
 #include <asm/hw_irq.h>
 #include <asm/stackprotector.h>
 #include <asm/sev.h>
+#include <asm/nospec-branch.h>
 
 /* representing HT siblings of each logical CPU */
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map);
@@ -1838,12 +1839,24 @@ void __noreturn hlt_play_dead(void)
 
 void native_play_dead(void)
 {
+	u64 spec_ctrl = spec_ctrl_current();
+
+	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
+		this_cpu_write(x86_spec_ctrl_current, 0);
+		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+	}
+
 	play_dead_common();
 	tboot_shutdown(TB_SHUTDOWN_WFS);
 
 	mwait_play_dead();
 	if (cpuidle_play_dead())
 		hlt_play_dead();
+
+	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
+		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
+		this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
+	}
 }
 
 #else /* ... !CONFIG_HOTPLUG_CPU */
-- 
2.31.1