[PATCH v1 1/3] x86/mm: change l1d flush runtime prctl behaviour

[Balbir Singh]

Detection of task affinities at API opt-in time is not the best
approach, the approach is to kill the task if it runs on a
SMT enable core. This is better than not flushing the L1D cache
when the task switches from a non-SMT core to an SMT enabled core.

Signed-off-by: Balbir Singh <sblbir@xxxxxxxxxx>
---
To be applied on top of tip commit id
b6724f118d44606fddde391ba7527526b3cad211

 arch/Kconfig          |  4 ++++
 arch/x86/Kconfig      |  1 +
 arch/x86/mm/tlb.c     | 37 +++++++++++++++++++++----------------
 include/linux/sched.h | 10 ++++++++++
 4 files changed, 36 insertions(+), 16 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 546869c3269d..2024486d28a2 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -311,6 +311,10 @@ config ARCH_32BIT_OFF_T
 	  still support 32-bit off_t. This option is enabled for all such
 	  architectures explicitly.
 
+config ARCH_HAS_PARANOID_L1D_FLUSH
+	bool
+	default n
+
 config HAVE_ASM_MODVERSIONS
 	bool
 	help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 36357c806e8a..02ff6ff71002 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -103,6 +103,7 @@ config X86
 	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_WANT_HUGE_PMD_SHARE
 	select ARCH_WANTS_THP_SWAP		if X86_64
+	select ARCH_HAS_PARANOID_L1D_FLUSH
 	select BUILDTIME_TABLE_SORT
 	select CLKEVT_I8253
 	select CLOCKSOURCE_VALIDATE_LAST_CYCLE
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 88e9ad5142e4..bdc399b86bc7 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -315,31 +315,18 @@ EXPORT_SYMBOL_GPL(leave_mm);
 
 int enable_l1d_flush_for_task(struct task_struct *tsk)
 {
-	int cpu, ret = 0, i;
-
 	/*
 	 * Do not enable L1D_FLUSH_OUT if
 	 * b. The CPU is not affected by the L1TF bug
 	 * c. The CPU does not have L1D FLUSH feature support
-	 * c. The task's affinity is on cores with SMT on.
 	 */
 
 	if (!boot_cpu_has_bug(X86_BUG_L1TF) ||
-			!static_cpu_has(X86_FEATURE_FLUSH_L1D))
+			!boot_cpu_has(X86_FEATURE_FLUSH_L1D))
 		return -EINVAL;
 
-	cpu = get_cpu();
-
-	for_each_cpu(i, &tsk->cpus_mask) {
-		if (cpu_data(i).smt_active == true) {
-			put_cpu();
-			return -EINVAL;
-		}
-	}
-
 	set_ti_thread_flag(&tsk->thread_info, TIF_SPEC_L1D_FLUSH);
-	put_cpu();
-	return ret;
+	return 0;
 }
 
 int disable_l1d_flush_for_task(struct task_struct *tsk)
@@ -358,13 +345,31 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	local_irq_restore(flags);
 }
 
+/*
+ * Sent to a task that opts into L1D flushing via the prctl interface
+ * but ends up running on an SMT enabled core.
+ */
+static void l1d_flush_kill(struct callback_head *ch)
+{
+	force_sig(SIGBUS);
+}
+
 static inline unsigned long mm_mangle_tif_spec_bits(struct task_struct *next)
 {
 	unsigned long next_tif = task_thread_info(next)->flags;
 	unsigned long spec_bits = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_SPEC_MASK;
+	unsigned long next_mm;
 
 	BUILD_BUG_ON(TIF_SPEC_L1D_FLUSH != TIF_SPEC_IB + 1);
-	return (unsigned long)next->mm | spec_bits;
+	next_mm = (unsigned long)next->mm | spec_bits;
+
+	if ((next_mm & LAST_USER_MM_L1D_FLUSH) && this_cpu_read(cpu_info.smt_active)) {
+		clear_ti_thread_flag(&next->thread_info, TIF_SPEC_L1D_FLUSH);
+		next->l1d_flush_kill.func = l1d_flush_kill;
+		task_work_add(next, &next->l1d_flush_kill, true);
+	}
+
+	return next_mm;
 }
 
 static void cond_mitigation(struct task_struct *next)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 322ea9b827e1..c569c49715d7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1338,6 +1338,16 @@ struct task_struct {
 	unsigned long			getblk_bh_state;
 #endif
 
+#ifdef CONFIG_ARCH_HAS_PARANOID_L1D_FLUSH
+	/*
+	 * If L1D flush is supported on mm context switch
+	 * then we use this callback head to queue kill work
+	 * to kill tasks that are not running on SMT disabled
+	 * cores
+	 */
+	struct callback_head		l1d_flush_kill;
+#endif
+
 	/*
 	 * New fields for task_struct should be added above here, so that
 	 * they are included in the randomized portion of task_struct.
-- 
2.17.1