[RFC 08/10] x86/idle: Control Indirect Branch Speculation in idle

[KarimAllah Ahmed]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Indirect Branch Speculation (IBS) is controlled per physical core. If one
thread disables it then it's disabled for the core. If a thread enters idle
it makes sense to reenable IBS so the sibling thread can run with full
speculation enabled in user space.

This makes only sense in mwait_idle_with_hints() because mwait_idle() can
serve an interrupt immediately before speculation can be stopped again. SKL
which requires IBRS should use mwait_idle_with_hints() so this is a non
issue and in the worst case a missed optimization.

Originally-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/mwait.h | 14 ++++++++++++++
 arch/x86/kernel/process.c    | 14 ++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 39a2fb2..f173072 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -6,6 +6,7 @@
 #include <linux/sched/idle.h>
 
 #include <asm/cpufeature.h>
+#include <asm/nospec-branch.h>
 
 #define MWAIT_SUBSTATE_MASK		0xf
 #define MWAIT_CSTATE_MASK		0xf
@@ -106,7 +107,20 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 			mb();
 		}
 
+		/*
+		 * Indirect Branch Speculation (IBS) is controlled per
+		 * physical core. If one thread disables it, then it's
+		 * disabled on all threads of the core. The kernel disables
+		 * it on entry from user space. Reenable it on the thread
+		 * which goes idle so the other thread has a chance to run
+		 * with full speculation enabled in userspace.
+		 */
+		unrestrict_branch_speculation();
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
+		/*
+		 * Restrict IBS again to protect kernel execution.
+		 */
+		restrict_branch_speculation();
 		if (!need_resched())
 			__mwait(eax, ecx);
 	}
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 3cb2486..f941c5d 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -461,6 +461,20 @@ static __cpuidle void mwait_idle(void)
 			mb(); /* quirk */
 		}
 
+		/*
+		 * Indirect Branch Speculation (IBS) is controlled per
+		 * physical core. If one thread disables it, then it's
+		 * disabled on all threads of the core. The kernel disables
+		 * it on entry from user space. For __sti_mwait() it's
+		 * wrong to reenable it because an interrupt can be served
+		 * before speculation can be stopped again.
+		 *
+		 * To plug that hole the interrupt entry code would need to
+		 * save current state and restore. Not worth the trouble as
+		 * SKL should not use mwait_idle(). It should use
+		 * mwait_idle_with_hints() which can do speculation control
+		 * safely.
+		 */
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
 		if (!need_resched())
 			__sti_mwait(0, 0);
-- 
2.7.4