Re: [PATCH v2 1/3] compiler_types: Introduce the Clang __preserve_most function attribute

[Peter Zijlstra]

On Fri, Aug 04, 2023 at 11:02:56AM +0200, Marco Elver wrote:
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 547ea1ff806e..12c4540335b7 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -106,6 +106,33 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
>  #define __cold
>  #endif
>  
> +/*
> + * On x86-64 and arm64 targets, __preserve_most changes the calling convention
> + * of a function to make the code in the caller as unintrusive as possible. This
> + * convention behaves identically to the C calling convention on how arguments
> + * and return values are passed, but uses a different set of caller- and callee-
> + * saved registers.
> + *
> + * The purpose is to alleviates the burden of saving and recovering a large
> + * register set before and after the call in the caller.  This is beneficial for
> + * rarely taken slow paths, such as error-reporting functions that may be called
> + * from hot paths.
> + *
> + * Note: This may conflict with instrumentation inserted on function entry which
> + * does not use __preserve_most or equivalent convention (if in assembly). Since
> + * function tracing assumes the normal C calling convention, where the attribute
> + * is supported, __preserve_most implies notrace.
> + *
> + * Optional: not supported by gcc.
> + *
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> + */
> +#if __has_attribute(__preserve_most__)
> +# define __preserve_most notrace __attribute__((__preserve_most__))
> +#else
> +# define __preserve_most
> +#endif

This seems to shrink the ARM64 vmlinux just a little and mirrors what we
do on x86 in asm. I'll leave it to the arm64 people to judge if this is
worth the hassle.

Index: linux-2.6/arch/arm64/include/asm/preempt.h
===================================================================
--- linux-2.6.orig/arch/arm64/include/asm/preempt.h
+++ linux-2.6/arch/arm64/include/asm/preempt.h
@@ -88,15 +88,18 @@ void preempt_schedule_notrace(void);
 #ifdef CONFIG_PREEMPT_DYNAMIC
 
 DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
-void dynamic_preempt_schedule(void);
+void __preserve_most dynamic_preempt_schedule(void);
 #define __preempt_schedule()		dynamic_preempt_schedule()
-void dynamic_preempt_schedule_notrace(void);
+void __preserve_most dynamic_preempt_schedule_notrace(void);
 #define __preempt_schedule_notrace()	dynamic_preempt_schedule_notrace()
 
 #else /* CONFIG_PREEMPT_DYNAMIC */
 
-#define __preempt_schedule()		preempt_schedule()
-#define __preempt_schedule_notrace()	preempt_schedule_notrace()
+void __preserve_most preserve_preempt_schedule(void);
+void __preserve_most preserve_preempt_schedule_notrace(void);
+
+#define __preempt_schedule()		preserve_preempt_schedule()
+#define __preempt_schedule_notrace()	preserve_preempt_schedule_notrace()
 
 #endif /* CONFIG_PREEMPT_DYNAMIC */
 #endif /* CONFIG_PREEMPTION */
Index: linux-2.6/kernel/sched/core.c
===================================================================
--- linux-2.6.orig/kernel/sched/core.c
+++ linux-2.6/kernel/sched/core.c
@@ -6915,7 +6915,7 @@ DEFINE_STATIC_CALL(preempt_schedule, pre
 EXPORT_STATIC_CALL_TRAMP(preempt_schedule);
 #elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
 static DEFINE_STATIC_KEY_TRUE(sk_dynamic_preempt_schedule);
-void __sched notrace dynamic_preempt_schedule(void)
+void __sched __preserve_most dynamic_preempt_schedule(void)
 {
 	if (!static_branch_unlikely(&sk_dynamic_preempt_schedule))
 		return;
@@ -6924,6 +6924,11 @@ void __sched notrace dynamic_preempt_sch
 NOKPROBE_SYMBOL(dynamic_preempt_schedule);
 EXPORT_SYMBOL(dynamic_preempt_schedule);
 #endif
+#else
+void __sched __preserve_most preserve_preempt_schedule(void)
+{
+	preempt_schedule();
+}
 #endif
 
 /**
@@ -6988,7 +6993,7 @@ DEFINE_STATIC_CALL(preempt_schedule_notr
 EXPORT_STATIC_CALL_TRAMP(preempt_schedule_notrace);
 #elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
 static DEFINE_STATIC_KEY_TRUE(sk_dynamic_preempt_schedule_notrace);
-void __sched notrace dynamic_preempt_schedule_notrace(void)
+void __sched __preserve_most dynamic_preempt_schedule_notrace(void)
 {
 	if (!static_branch_unlikely(&sk_dynamic_preempt_schedule_notrace))
 		return;
@@ -6997,6 +7002,11 @@ void __sched notrace dynamic_preempt_sch
 NOKPROBE_SYMBOL(dynamic_preempt_schedule_notrace);
 EXPORT_SYMBOL(dynamic_preempt_schedule_notrace);
 #endif
+#else
+void __sched __preserve_most preserve_preempt_schedule_notrace(void)
+{
+	preempt_schedule_notrace();
+}
 #endif
 
 #endif /* CONFIG_PREEMPTION */