linux-kernel - Re: [PATCH v10 11/16] entry: Split syscall_exit_to_user_mode

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Message-ID: <f3c32e7f-f940-4cb0-941c-cb7f7f628d88@arm.com>
Date: Mon, 29 Dec 2025 19:03:35 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Jinjie Ruan <ruanjinjie@...wei.com>, catalin.marinas@....com,
 will@...nel.org, oleg@...hat.com, tglx@...utronix.de, peterz@...radead.org,
 luto@...nel.org, shuah@...nel.org, kees@...nel.org, wad@...omium.org,
 macro@...am.me.uk, charlie@...osinc.com, akpm@...ux-foundation.org,
 ldv@...ace.io, anshuman.khandual@....com, mark.rutland@....com,
 thuth@...hat.com, song@...nel.org, ryan.roberts@....com,
 ada.coupriediaz@....com, broonie@...nel.org, liqiang01@...inos.cn,
 pengcan@...inos.cn, kmal@...k.li, dvyukov@...gle.com,
 richard.weiyang@...il.com, reddybalavignesh9979@...il.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v10 11/16] entry: Split syscall_exit_to_user_mode_work()
 for arch reuse

On 22/12/2025 12:47, Jinjie Ruan wrote:
> In the generic entry code, the beginning of
> syscall_exit_to_user_mode_work() can be reused on arm64 so it makes
> sense to split it.
>
> In preparation for moving arm64 over to the generic entry
> code, split out syscall_exit_to_user_mode_work_prepare() helper from
> syscall_exit_to_user_mode_work().
>
> No functional changes.
>
> Reviewed-by: Kevin Brodsky <kevin.brodsky@....com>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Jinjie Ruan <ruanjinjie@...wei.com>
> ---
>  include/linux/entry-common.h | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index 87efb38b7081..0de0e60630e1 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -121,20 +121,11 @@ static __always_inline long syscall_enter_from_user_mode(struct pt_regs *regs, l
>   */
>  void syscall_exit_work(struct pt_regs *regs, unsigned long work);
>  
> -/**
> - * syscall_exit_to_user_mode_work - Handle work before returning to user mode
> - * @regs:	Pointer to currents pt_regs
> - *
> - * Same as step 1 and 2 of syscall_exit_to_user_mode() but without calling
> - * exit_to_user_mode() to perform the final transition to user mode.
> - *
> - * Calling convention is the same as for syscall_exit_to_user_mode() and it
> - * returns with all work handled and interrupts disabled. The caller must
> - * invoke exit_to_user_mode() before actually switching to user mode to
> - * make the final state transitions. Interrupts must stay disabled between
> - * return from this function and the invocation of exit_to_user_mode().
> +/*
> + * Syscall specific exit to user mode preparation. Runs with interrupts
> + * enabled.
>   */
> -static __always_inline void syscall_exit_to_user_mode_work(struct pt_regs *regs)
> +static __always_inline void syscall_exit_to_user_mode_work_prepare(struct pt_regs *regs)
>  {
>  	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
>  	unsigned long nr = syscall_get_nr(current, regs);
> @@ -155,6 +146,24 @@ static __always_inline void syscall_exit_to_user_mode_work(struct pt_regs *regs)
>  	 */
>  	if (unlikely(work & SYSCALL_WORK_EXIT))
>  		syscall_exit_work(regs, work);
> +}
> +
> +/**
> + * syscall_exit_to_user_mode_work - Handle work before returning to user mode
> + * @regs:	Pointer to currents pt_regs
> + *
> + * Same as step 1 and 2 of syscall_exit_to_user_mode() but without calling
> + * exit_to_user_mode() to perform the final transition to user mode.
> + *
> + * Calling convention is the same as for syscall_exit_to_user_mode() and it
> + * returns with all work handled and interrupts disabled. The caller must
> + * invoke exit_to_user_mode() before actually switching to user mode to
> + * make the final state transitions. Interrupts must stay disabled between
> + * return from this function and the invocation of exit_to_user_mode().
> + */
> +static __always_inline void syscall_exit_to_user_mode_work(struct pt_regs *regs)
> +{
> +	syscall_exit_to_user_mode_work_prepare(regs);

The naming is getting awfully confusing, with the separate introduction
of syscall_exit_to_user_mode_prepare().

Having had a closer look, do we really need
syscall_exit_to_user_mode_work() as it currently stands? Nothing calls
it except the generic syscall_exit_to_user_mode(). Which makes me think:
how about moving the two lines below into syscall_exit_to_user_mode()
instead of creating a new helper? IOW:

@@ -155,8 +155,6 @@ static __always_inline void
syscall_exit_to_user_mode_work(struct pt_regs *regs)
      */
     if (unlikely(work & SYSCALL_WORK_EXIT))
         syscall_exit_work(regs, work);
-    local_irq_disable_exit_to_user();
-    syscall_exit_to_user_mode_prepare(regs);
 }
 
 /**
@@ -192,6 +190,8 @@ static __always_inline void
syscall_exit_to_user_mode(struct pt_regs *regs)
 {
     instrumentation_begin();
     syscall_exit_to_user_mode_work(regs);
+    local_irq_disable_exit_to_user();
+    syscall_exit_to_user_mode_prepare(regs);
     instrumentation_end();
     exit_to_user_mode();
 }

- Kevin

>  	local_irq_disable_exit_to_user();
>  	syscall_exit_to_user_mode_prepare(regs);
>  }