Re: [patch 02/12] rseq: Add fields and constants for time slice extension

Mathieu Desnoyers
2025-09-11T15:49:31Z

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.