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

Prakash Sangappa
2025-09-23T00:58:06Z

> On Sep 22, 2025, at 6:55 AM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>
> On 2025-09-22 01:28, Prakash Sangappa wrote:
>>> On Sep 8, 2025, at 3:59 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>>>
>> ..
>>> +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;
>> We intend to backport the slice extension feature to older kernel versions.
>> With use of a new structure member for slice control, could there be discrepancy
>> with rseq structure size(older version) registered by libc? In that case the application
>> may not be able to use slice extension feature unless Libc’s use of rseq is disabled.
>
> The rseq extension scheme allows this to seamlessly work.
>
> You will need a glibc 2.41+, which uses the getauxval(3)
> AT_RSEQ_FEATURE_SIZE and AT_RSEQ_ALIGN to query the feature size
> supported by the Linux kernel. It allocates a per-thread memory
> area which is large enough to support that feature set, and
> registers it to the kernel through rseq(2) on thread creation.

Ok,

>
> Note that before we had the extensible rseq scheme, glibc registered
> a 32-byte structure (including padding at the end), which is considered
> as the rseq "original" registration size.
>
> The "mm_cid" field ends at 28 bytes, which leaves 4 bytes of padding at
> the end of the original rseq structure. Considering that the time slice
> extension fields will likely fit within those 4 bytes, I expect that
> applications linked against glibc [2.35, 2.40] will also be able to use
> those fields. Those applications should use getauxval(3)
> AT_RSEQ_FEATURE_SIZE to validate whether the kernel populates this field
> or if it's just padding.

The question was about the size of rseq structure registered by glibc. If it is using
AT_RSEQ_FEATURE_SIZE to allocate the per-thread area for rseq, I suppose that
should be fine. However application would have to verify that __rseq_size size is large
enough.

As for the Kernel supporting slice extension, I expect the prctl(..,PR_RSEQ_SLICE_EXT_ENABLE)
would return an error if it is not supported, won’t that be sufficient or should it check
AT_RSEQ_FEATURE_SIZE?

>
> Note that this all works even if you backport the feature to an older kernel:
> the rseq extension scheme does not depend on querying the kernel version at
> all. You will however be required to backport the support for additional
> rseq fields that come before the time slice, such as node_id and mm_cid,
> if they are not implemented in your older kernel.

Yes, need to look at those changes that needs to be backported. Also, the dependent
'rseq: Optimize exit to user space’ changes from other patch series.

>
>> Application would have to verify structure size, so should it be mentioned in the
>> documentation.
>
> Yes, applications should check that the glibc's __rseq_size is large enough to fit
> the new slice field(s), *and* for the original rseq size special case
> (32 bytes including padding), those would need to query getauxval(3)
> AT_RSEQ_FEATURE_SIZE to make sure the field is indeed supported.
>
> Also, perhaps make the prctl() enable call return error, if structure size
>> does not match?
>
> That's not how the extensible scheme works.
>
> Either glibc registers a 32-byte area (in which the time slice feature would
> fit), or it registers an area large enough to fit all kernel supported features,
> or it fails registration. And prctl() is per-process, whereas the rseq registration
> is per-thread, so it's kind of weird to make prctl() fail if the current
> thread's rseq is not registered.

I meant the prctl(.., PR_RSEQ_SLICE_EXT_ENABLE) call is per thread and
sets the enabled bit in per thread rseq. This could fail if rseq struct size is not large enough?

>
>> With regards to application determining the address and size of rseq structure
>> registered by libc, what are you thoughts on getting that thru the rseq(2)
>> system call or a prctl() call instead of dealing with the __week symbols as was discussed here.
>> https://lore.kernel.org/all/F9DBABAD-ABF0-49AA-9A38-BD4D2BE78B94@oracle.com/
>
> I think that the other leg of that email thread got to a resolution of both static and
> dynamic use-cases through use of an extern __weak symbol, no [1] ? Not that I am against
> adding a rseq(2) query for rseq address, size, and signature, but I just want to double
> check that it would be there for convenience and is not actually needed in the typical
> use-cases.

Yes, mainly for convenience.

Thanks,
-Prakash

>
> Thanks,
>
> Mathieu
>
> [1] https://lore.kernel.org/all/aKPFIQwg5zxSS5oS@google.com/
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com