x86/calldepth: Add ret/call counting for debug

Add a debuigfs mechanism to validate the accounting, e.g. vs. call/ret
balance and to gather statistics about the stuffing to call ratio.

Signed-off-by: Thomas Gleixner <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/nospec-branch.h

···
 #define RET_DEPTH_INIT_FROM_CALL	0xfc00000000000000ULL
 #define RET_DEPTH_CREDIT		0xffffffffffffffffULL
 
+#ifdef CONFIG_CALL_THUNKS_DEBUG
+# define CALL_THUNKS_DEBUG_INC_CALLS				\
+	incq	%gs:__x86_call_count;
+# define CALL_THUNKS_DEBUG_INC_RETS				\
+	incq	%gs:__x86_ret_count;
+# define CALL_THUNKS_DEBUG_INC_STUFFS				\
+	incq	%gs:__x86_stuffs_count;
+# define CALL_THUNKS_DEBUG_INC_CTXSW				\
+	incq	%gs:__x86_ctxsw_count;
+#else
+# define CALL_THUNKS_DEBUG_INC_CALLS
+# define CALL_THUNKS_DEBUG_INC_RETS
+# define CALL_THUNKS_DEBUG_INC_STUFFS
+# define CALL_THUNKS_DEBUG_INC_CTXSW
+#endif
+
 #if defined(CONFIG_CALL_DEPTH_TRACKING) && !defined(COMPILE_OFFSETS)
 
 #include <asm/asm-offsets.h>
···
 #define RESET_CALL_DEPTH_FROM_CALL				\
 	mov	$0xfc, %rax;					\
 	shl	$56, %rax;					\
-	movq	%rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);
+	movq	%rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);	\
+	CALL_THUNKS_DEBUG_INC_CALLS
 
 #define INCREMENT_CALL_DEPTH					\
-	sarq	$5, %gs:pcpu_hot + X86_call_depth;
+	sarq	$5, %gs:pcpu_hot + X86_call_depth;		\
+	CALL_THUNKS_DEBUG_INC_CALLS
 
 #define ASM_INCREMENT_CALL_DEPTH				\
-	sarq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth);
+	sarq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth);	\
+	CALL_THUNKS_DEBUG_INC_CALLS
 
 #else
 #define CREDIT_CALL_DEPTH
+#define ASM_CREDIT_CALL_DEPTH
 #define RESET_CALL_DEPTH
 #define INCREMENT_CALL_DEPTH
+#define ASM_INCREMENT_CALL_DEPTH
 #define RESET_CALL_DEPTH_FROM_CALL
 #endif
 
···
 	jnz	771b;					\
 	/* barrier for jnz misprediction */		\
 	lfence;						\
-	ASM_CREDIT_CALL_DEPTH
+	ASM_CREDIT_CALL_DEPTH				\
+	CALL_THUNKS_DEBUG_INC_CTXSW
 #else
 /*
  * i386 doesn't unconditionally have LFENCE, as such it can't
···
 {
 	x86_return_thunk = &__x86_return_skl;
 }
+#ifdef CONFIG_CALL_THUNKS_DEBUG
+DECLARE_PER_CPU(u64, __x86_call_count);
+DECLARE_PER_CPU(u64, __x86_ret_count);
+DECLARE_PER_CPU(u64, __x86_stuffs_count);
+DECLARE_PER_CPU(u64, __x86_ctxsw_count);
+#endif
 #else
 static inline void x86_set_skl_return_thunk(void) {}
 #endif

arch/x86/kernel/callthunks.c

···
 
 #define pr_fmt(fmt) "callthunks: " fmt
 
+#include <linux/debugfs.h>
 #include <linux/kallsyms.h>
 #include <linux/memory.h>
 #include <linux/moduleloader.h>
···
 	return 1;
 }
 __setup("debug-callthunks", debug_thunks);
+
+#ifdef CONFIG_CALL_THUNKS_DEBUG
+DEFINE_PER_CPU(u64, __x86_call_count);
+DEFINE_PER_CPU(u64, __x86_ret_count);
+DEFINE_PER_CPU(u64, __x86_stuffs_count);
+DEFINE_PER_CPU(u64, __x86_ctxsw_count);
+EXPORT_SYMBOL_GPL(__x86_ctxsw_count);
+EXPORT_SYMBOL_GPL(__x86_call_count);
+#endif
 
 extern s32 __call_sites[], __call_sites_end[];
 
···
 	mutex_unlock(&text_mutex);
 }
 #endif /* CONFIG_MODULES */
+
+#if defined(CONFIG_CALL_THUNKS_DEBUG) && defined(CONFIG_DEBUG_FS)
+static int callthunks_debug_show(struct seq_file *m, void *p)
+{
+	unsigned long cpu = (unsigned long)m->private;
+
+	seq_printf(m, "C: %16llu R: %16llu S: %16llu X: %16llu\n,",
+		   per_cpu(__x86_call_count, cpu),
+		   per_cpu(__x86_ret_count, cpu),
+		   per_cpu(__x86_stuffs_count, cpu),
+		   per_cpu(__x86_ctxsw_count, cpu));
+	return 0;
+}
+
+static int callthunks_debug_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, callthunks_debug_show, inode->i_private);
+}
+
+static const struct file_operations dfs_ops = {
+	.open		= callthunks_debug_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+static int __init callthunks_debugfs_init(void)
+{
+	struct dentry *dir;
+	unsigned long cpu;
+
+	dir = debugfs_create_dir("callthunks", NULL);
+	for_each_possible_cpu(cpu) {
+		void *arg = (void *)cpu;
+		char name [10];
+
+		sprintf(name, "cpu%lu", cpu);
+		debugfs_create_file(name, 0644, dir, arg, &dfs_ops);
+	}
+	return 0;
+}
+__initcall(callthunks_debugfs_init);
+#endif

arch/x86/lib/retpoline.S

···
 	.align 64
 SYM_FUNC_START(__x86_return_skl)
 	ANNOTATE_NOENDBR
-	/* Keep the hotpath in a 16byte I-fetch */
+	/*
+	 * Keep the hotpath in a 16byte I-fetch for the non-debug
+	 * case.
+	 */
+	CALL_THUNKS_DEBUG_INC_RETS
 	shlq	$5, PER_CPU_VAR(pcpu_hot + X86_call_depth)
 	jz	1f
 	ANNOTATE_UNRET_SAFE
 	ret
 	int3
 1:
+	CALL_THUNKS_DEBUG_INC_STUFFS
 	.rept	16
 	ANNOTATE_INTRA_FUNCTION_CALL
 	call	2f