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

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Message-ID: <fd5abd03-0198-44cb-8cfd-b0d2cf44a35d@efficios.com>
Date: Mon, 22 Sep 2025 09:57:30 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: K Prateek Nayak <kprateek.nayak@....com>,
 Prakash Sangappa <prakash.sangappa@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra
 <peterz@...radead.org>, "Paul E. McKenney" <paulmck@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Jonathan Corbet <corbet@....net>,
 Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Arnd Bergmann <arnd@...db.de>,
 "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [patch 02/12] rseq: Add fields and constants for time slice
 extension

On 2025-09-22 01:57, K Prateek Nayak wrote:
> Hello Prakash,
> 
> On 9/22/2025 10:58 AM, Prakash Sangappa wrote:
>> 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.
> 
> In this case, wouldn't GLIBC's rseq registration fail if presumed
> __rseq_size is smaller than the "struct rseq" size?

The registered rseq size cannot be smaller than 32 bytes, else
registration is refused by the system call (-EINVAL).

The new slice extension fields would fit within those 32 bytes,
so it should always work.

Thanks,

Mathieu

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