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Message-ID: <010dbadb-0eef-4a35-95c1-932ad26cff21@efficios.com>
Date: Thu, 11 Sep 2025 11:49:24 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
"Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
Jonathan Corbet <corbet@....net>,
Prakash Sangappa <prakash.sangappa@...cle.com>,
Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
K Prateek Nayak <kprateek.nayak@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
Michael Jeanson <mjeanson@...icios.com>
Subject: Re: [patch 02/12] rseq: Add fields and constants for time slice
extension
On 2025-09-11 11:41, Mathieu Desnoyers wrote:
> On 2025-09-08 18:59, Thomas Gleixner wrote:
[...]
>
>> +
>> +The kernel enforces flag consistency and terminates the thread with
>> SIGSEGV
>> +if it detects a violation.
>> --- a/include/linux/rseq_types.h
>> +++ b/include/linux/rseq_types.h
>> @@ -71,12 +71,35 @@ struct rseq_ids {
>> };
>> /**
>> + * union rseq_slice_state - Status information for rseq time slice
>> extension
>> + * @state: Compound to access the overall state
>> + * @enabled: Time slice extension is enabled for the task
>> + * @granted: Time slice extension was granted to the task
>> + */
>> +union rseq_slice_state {
>> + u16 state;
>> + struct {
>> + u8 enabled;
>> + u8 granted;
>> + };
>> +};
>> +
>> +/**
>> + * struct rseq_slice - Status information for rseq time slice extension
>> + * @state: Time slice extension state
>> + */
>> +struct rseq_slice {
>> + union rseq_slice_state state;
>> +};
>> +
>> +/**
>> * struct rseq_data - Storage for all rseq related data
>> * @usrptr: Pointer to the registered user space RSEQ memory
>> * @len: Length of the RSEQ region
>> * @sig: Signature of critial section abort IPs
>> * @event: Storage for event management
>> * @ids: Storage for cached CPU ID and MM CID
>> + * @slice: Storage for time slice extension data
>> */
>> struct rseq_data {
>> struct rseq __user *usrptr;
>> @@ -84,6 +107,9 @@ struct rseq_data {
>> u32 sig;
>> struct rseq_event event;
>> struct rseq_ids ids;
>> +#ifdef CONFIG_RSEQ_SLICE_EXTENSION
>> + struct rseq_slice slice;
>> +#endif
Note: we could move this #ifdef to surround the definition
of both union rseq_slice_state and struct rseq_slice,
and emit an empty structure in the #else case rather than
do the ifdef here.
Thanks,
Mathieu
>> };
>> #else /* CONFIG_RSEQ */
>> --- a/include/uapi/linux/rseq.h
>> +++ b/include/uapi/linux/rseq.h
>> @@ -23,9 +23,15 @@ enum rseq_flags {
>> };
>> enum rseq_cs_flags_bit {
>> + /* Historical and unsupported bits */
>> RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT = 0,
>> RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT = 1,
>> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT = 2,
>> + /* (3) Intentional gap to put new bits into a seperate byte */
>> +
>> + /* User read only feature flags */
>> + RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT = 4,
>> + RSEQ_CS_FLAG_SLICE_EXT_ENABLED_BIT = 5,
>> };
>> enum rseq_cs_flags {
>> @@ -35,6 +41,22 @@ enum rseq_cs_flags {
>> (1U << RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT),
>> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE =
>> (1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT),
>> +
>> + RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE =
>> + (1U << RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE_BIT),
>> + RSEQ_CS_FLAG_SLICE_EXT_ENABLED =
>> + (1U << RSEQ_CS_FLAG_SLICE_EXT_ENABLED_BIT),
>> +};
>> +
>> +enum rseq_slice_bits {
>> + /* Time slice extension ABI bits */
>> + RSEQ_SLICE_EXT_REQUEST_BIT = 0,
>> + RSEQ_SLICE_EXT_GRANTED_BIT = 1,
>> +};
>> +
>> +enum rseq_slice_masks {
>> + RSEQ_SLICE_EXT_REQUEST = (1U << RSEQ_SLICE_EXT_REQUEST_BIT),
>> + RSEQ_SLICE_EXT_GRANTED = (1U << RSEQ_SLICE_EXT_GRANTED_BIT),
>> };
>> /*
>> @@ -142,6 +164,12 @@ struct rseq {
>> __u32 mm_cid;
>> /*
>> + * Time slice extension control word. CPU local atomic updates from
>> + * kernel and user space.
>> + */
>> + __u32 slice_ctrl;
>> +
>> + /*
>> * Flexible array member at end of structure, after last feature
>> field.
>> */
>> char end[];
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1908,6 +1908,18 @@ config RSEQ_DEBUG_DEFAULT_ENABLE
>> If unsure, say N.
>> +config RSEQ_SLICE_EXTENSION
>> + bool "Enable rseq based time slice extension mechanism"
>> + depends on RSEQ && HIGH_RES_TIMERS && GENERIC_ENTRY &&
>> HAVE_GENERIC_TIF_BITS
>> + help
>> + Allows userspace to request a limited time slice extension
>> when
>> + returning from an interrupt to user space via the RSEQ shared
>> + data ABI. If granted, that allows to complete a critical section,
>> + so that other threads are not stuck on a conflicted resource,
>> + while the task is scheduled out.
>> +
>> + If unsure, say N.
>> +
>> config DEBUG_RSEQ
>> default n
>> bool "Enable debugging of rseq() system call" if EXPERT
>> --- a/kernel/rseq.c
>> +++ b/kernel/rseq.c
>> @@ -387,6 +387,8 @@ static bool rseq_reset_ids(void)
>> */
>> SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
>> int, flags, u32, sig)
>> {
>> + u32 rseqfl = 0;
>> +
>> if (flags & RSEQ_FLAG_UNREGISTER) {
>> if (flags & ~RSEQ_FLAG_UNREGISTER)
>> return -EINVAL;
>> @@ -448,6 +450,12 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
>> if (put_user_masked_u64(0UL, &rseq->rseq_cs))
>> return -EFAULT;
>> + if (IS_ENABLED(CONFIG_RSEQ_SLICE_EXTENSION))
>> + rseqfl |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
>> +
>> + if (put_user_masked_u32(rseqfl, &rseq->flags))
>> + return -EFAULT;
>> +
>> /*
>> * Activate the registration by setting the rseq area address,
>> length
>> * and signature in the task struct.
>>
>
>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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