linux-kernel - Re: [patch V2 15/37] rseq: Record interrupt from user space

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Message-ID: <775c7f2e-975d-4262-964f-7d2ed0554fad@efficios.com>
Date: Mon, 25 Aug 2025 14:29:55 -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 15/37] rseq: Record interrupt from user space

On 2025-08-23 12:39, Thomas Gleixner wrote:
> For RSEQ the only relevant reason to inspect and eventually fixup (abort)
> user space critical sections is when user space was interrupted and the
> task was scheduled out.
> 
> If the user to kernel entry was from a syscall no fixup is required. If
> user space invokes a syscall from a critical section it can keep the
> pieces as documented.
> 
> This is only supported on architectures, which utilize the generic entry

no comma between "architectures" and "which".

> code. If your architecture does not use it, bad luck.
> 

Should we eventually add a "depends on GENERIC_IRQ_ENTRY" to RSEQ then ?

> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>   include/linux/irq-entry-common.h |    3 ++-
>   include/linux/rseq.h             |   16 +++++++++++-----
>   include/linux/rseq_entry.h       |   18 ++++++++++++++++++
>   include/linux/rseq_types.h       |    2 ++
>   4 files changed, 33 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -4,7 +4,7 @@
>   
>   #include <linux/context_tracking.h>
>   #include <linux/kmsan.h>
> -#include <linux/rseq.h>
> +#include <linux/rseq_entry.h>
>   #include <linux/static_call_types.h>
>   #include <linux/syscalls.h>
>   #include <linux/tick.h>
> @@ -281,6 +281,7 @@ static __always_inline void exit_to_user
>   static __always_inline void irqentry_enter_from_user_mode(struct pt_regs *regs)
>   {
>   	enter_from_user_mode(regs);
> +	rseq_note_user_irq_entry();

As long as this also covers the following scenarios I'm ok with this:

- trap/exception from an rseq critical section,
- NMI over an rseq critical section.

Thanks,

Mathieu

>   }
>   
>   /**
> --- a/include/linux/rseq.h
> +++ b/include/linux/rseq.h
> @@ -31,11 +31,17 @@ static inline void rseq_sched_switch_eve
>   
>   static __always_inline void rseq_exit_to_user_mode(void)
>   {
> -	if (IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
> -		if (WARN_ON_ONCE(current->rseq_event.has_rseq &&
> -				 current->rseq_event.events))
> -			current->rseq_event.events = 0;
> -	}
> +	struct rseq_event *ev = &current->rseq_event;
> +
> +	if (IS_ENABLED(CONFIG_DEBUG_RSEQ))
> +		WARN_ON_ONCE(ev->sched_switch);
> +
> +	/*
> +	 * Ensure that event (especially user_irq) is cleared when the
> +	 * interrupt did not result in a schedule and therefore the
> +	 * rseq processing did not clear it.
> +	 */
> +	ev->events = 0;
>   }
>   
>   /*
> --- /dev/null
> +++ b/include/linux/rseq_entry.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_RSEQ_ENTRY_H
> +#define _LINUX_RSEQ_ENTRY_H
> +
> +#ifdef CONFIG_RSEQ
> +#include <linux/rseq.h>
> +
> +static __always_inline void rseq_note_user_irq_entry(void)
> +{
> +	if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))
> +		current->rseq_event.user_irq = true;
> +}
> +
> +#else /* CONFIG_RSEQ */
> +static inline void rseq_note_user_irq_entry(void) { }
> +#endif /* !CONFIG_RSEQ */
> +
> +#endif /* _LINUX_RSEQ_ENTRY_H */
> --- a/include/linux/rseq_types.h
> +++ b/include/linux/rseq_types.h
> @@ -9,6 +9,7 @@
>    * @all:		Compound to initialize and clear the data efficiently
>    * @events:		Compund to access events with a single load/store
>    * @sched_switch:	True if the task was scheduled out
> + * @user_irq:		True on interrupt entry from user mode
>    * @has_rseq:		True if the task has a rseq pointer installed
>    */
>   struct rseq_event {
> @@ -19,6 +20,7 @@ struct rseq_event {
>   				u16		events;
>   				struct {
>   					u8	sched_switch;
> +					u8	user_irq;
>   				};
>   			};
>   
> 


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