Re: [patch V4 12/36] entry: Inline irqentry_enter/exit_from/to_user_mode()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-09-08 17:31, Thomas Gleixner wrote:
> There is no point to have this as a function which just inlines
> enter_from_user_mode(). The function call overhead is larger than the
> function itself.

I'm wondering if there is a reason for making those actual functions and
not inlines.

The functions sit in kernel/entry/common.c, which are built with
specific compiler flags in kernel/entry/Makefile:

# Prevent the noinstr section from being pestered by sanitizer and other 
goodies
# as long as these things cannot be disabled per function.
KASAN_SANITIZE := n
UBSAN_SANITIZE := n
KCOV_INSTRUMENT := n

# Branch profiling isn't noinstr-safe
ccflags-$(CONFIG_TRACE_BRANCH_PROFILING) += -DDISABLE_BRANCH_PROFILING

CFLAGS_REMOVE_common.o   = -fstack-protector -fstack-protector-strong
CFLAGS_common.o         += -fno-stack-protector

So I wonder if we're not breaking something in the area of "noinstr"
tagging by inlining those into their caller ?

Thanks,

Mathieu

> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 
> ---
>   include/linux/irq-entry-common.h |   13 +++++++++++--
>   kernel/entry/common.c            |   13 -------------
>   2 files changed, 11 insertions(+), 15 deletions(-)
> 
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -278,7 +278,10 @@ static __always_inline void exit_to_user
>    *
>    * The function establishes state (lockdep, RCU (context tracking), tracing)
>    */
> -void irqentry_enter_from_user_mode(struct pt_regs *regs);
> +static __always_inline void irqentry_enter_from_user_mode(struct pt_regs *regs)
> +{
> +	enter_from_user_mode(regs);
> +}
>   
>   /**
>    * irqentry_exit_to_user_mode - Interrupt exit work
> @@ -293,7 +296,13 @@ void irqentry_enter_from_user_mode(struc
>    * Interrupt exit is not invoking #1 which is the syscall specific one time
>    * work.
>    */
> -void irqentry_exit_to_user_mode(struct pt_regs *regs);
> +static __always_inline void irqentry_exit_to_user_mode(struct pt_regs *regs)
> +{
> +	instrumentation_begin();
> +	exit_to_user_mode_prepare(regs);
> +	instrumentation_end();
> +	exit_to_user_mode();
> +}
>   
>   #ifndef irqentry_state
>   /**
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -62,19 +62,6 @@ void __weak arch_do_signal_or_restart(st
>   	return ti_work;
>   }
>   
> -noinstr void irqentry_enter_from_user_mode(struct pt_regs *regs)
> -{
> -	enter_from_user_mode(regs);
> -}
> -
> -noinstr void irqentry_exit_to_user_mode(struct pt_regs *regs)
> -{
> -	instrumentation_begin();
> -	exit_to_user_mode_prepare(regs);
> -	instrumentation_end();
> -	exit_to_user_mode();
> -}
> -
>   noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
>   {
>   	irqentry_state_t ret = {
> 
> 


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com