[PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline

[Waiman Long <longman@redhat.com>]

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. When the CPU is online again, it will be re-initialized
and so restoring the SPEC_CTRL value isn't needed.

Add a comment to say that native_play_dead() is a __noreturn function,
but it can't be marked as such to avoid confusion about the missing
MSR restoration code.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 arch/x86/kernel/smpboot.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 352f0ce1ece4..7bc33885518c 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);
@@ -1836,8 +1837,17 @@ void __noreturn hlt_play_dead(void)
 	}
 }
 
+/*
+ * naitve_play_dead() is essentially a __noreturn function, but it can't
+ * be marked as such as the compiler may complain about it.
+ */
 void native_play_dead(void)
 {
+	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);
 
-- 
2.31.1