linux-kernel - Re: [patch V2 29/37] entry: Split up exit_to_user_mode

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Message-ID: <9ac25827-6421-413c-86d1-09f877b6e651@efficios.com>
Date: Tue, 26 Aug 2025 11:41:50 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 29/37] entry: Split up exit_to_user_mode_prepare()

On 2025-08-23 12:40, Thomas Gleixner wrote:
> exit_to_user_mode_prepare() is used for both interrupts and syscalls, but
> there is extra rseq work, which is only required for in the interrupt exit
> case.
> 
> Split up the function and provide wrappers for syscalls and interrupts,
> which allows to seperate the rseq exit work in the next step.

separate

> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>   include/linux/entry-common.h     |    2 -
>   include/linux/irq-entry-common.h |   42 ++++++++++++++++++++++++++++++++++-----
>   2 files changed, 38 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -156,7 +156,7 @@ static __always_inline void syscall_exit
>   	if (unlikely(work & SYSCALL_WORK_EXIT))
>   		syscall_exit_work(regs, work);
>   	local_irq_disable_exit_to_user();
> -	exit_to_user_mode_prepare(regs);
> +	syscall_exit_to_user_mode_prepare(regs);
>   }
>   
>   /**
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -201,7 +201,7 @@ void arch_do_signal_or_restart(struct pt
>   unsigned long exit_to_user_mode_loop(struct pt_regs *regs, unsigned long ti_work);
>   
>   /**
> - * exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
> + * __exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
>    * @regs:	Pointer to pt_regs on entry stack
>    *
>    * 1) check that interrupts are disabled
> @@ -209,8 +209,10 @@ unsigned long exit_to_user_mode_loop(str
>    * 3) call exit_to_user_mode_loop() if any flags from
>    *    EXIT_TO_USER_MODE_WORK are set
>    * 4) check that interrupts are still disabled
> + *
> + * Don't invoke directly, use the syscall/irqentry_ prefixed variants below
>    */
> -static __always_inline void exit_to_user_mode_prepare(struct pt_regs *regs)
> +static __always_inline void __exit_to_user_mode_prepare(struct pt_regs *regs)
>   {
>   	unsigned long ti_work;
>   
> @@ -224,15 +226,45 @@ static __always_inline void exit_to_user
>   		ti_work = exit_to_user_mode_loop(regs, ti_work);
>   
>   	arch_exit_to_user_mode_prepare(regs, ti_work);
> +}
>   
> -	rseq_exit_to_user_mode();
> -
> +static __always_inline void __exit_to_user_mode_validate(void)
> +{
>   	/* Ensure that kernel state is sane for a return to userspace */
>   	kmap_assert_nomap();
>   	lockdep_assert_irqs_disabled();
>   	lockdep_sys_exit();
>   }
>   
> +
> +/**
> + * syscall_exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
> + * @regs:	Pointer to pt_regs on entry stack
> + *
> + * Wrapper around __exit_to_user_mode_prepare() to seperate the exit work for

separate

> + * syscalls and interrupts.
> + */
> +static __always_inline void syscall_exit_to_user_mode_prepare(struct pt_regs *regs)
> +{
> +	__exit_to_user_mode_prepare(regs);
> +	rseq_exit_to_user_mode();
> +	__exit_to_user_mode_validate();
> +}
> +
> +/**
> + * irqentry_exit_to_user_mode_prepare - call exit_to_user_mode_loop() if required
> + * @regs:	Pointer to pt_regs on entry stack
> + *
> + * Wrapper around __exit_to_user_mode_prepare() to seperate the exit work for

separate

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> + * syscalls and interrupts.
> + */
> +static __always_inline void irqentry_exit_to_user_mode_prepare(struct pt_regs *regs)
> +{
> +	__exit_to_user_mode_prepare(regs);
> +	rseq_exit_to_user_mode();
> +	__exit_to_user_mode_validate();
> +}
> +
>   /**
>    * exit_to_user_mode - Fixup state when exiting to user mode
>    *
> @@ -297,7 +329,7 @@ static __always_inline void irqentry_ent
>   static __always_inline void irqentry_exit_to_user_mode(struct pt_regs *regs)
>   {
>   	instrumentation_begin();
> -	exit_to_user_mode_prepare(regs);
> +	irqentry_exit_to_user_mode_prepare(regs);
>   	instrumentation_end();
>   	exit_to_user_mode();
>   }
> 


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