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Tue, 24 Feb 2026 16:02:10 +0000 (GMT) Date: Tue, 24 Feb 2026 17:02:08 +0100 From: Heiko Carstens To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , mathieu.desnoyers@efficios.com, Mark Rutland , cmarinas@kernel.org, maddy@linux.ibm.com, ryan.roberts@arm.com Subject: Re: [RFC] in-kernel rseq Message-ID: <20260224160208.20006F04-hca@linux.ibm.com> References: <20260223163843.GR1282955@noisy.programming.kicks-ass.net> <20260224111646.20006Ddc-hca@linux.ibm.com> <20260224152032.GZ1395266@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260224152032.GZ1395266@noisy.programming.kicks-ass.net> X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: iyS6-Hv6LPaviWCj5CG_MQcSN9j2j3tb X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMjI0MDEyOSBTYWx0ZWRfX11VJV7vNnTT7 PXENvsHWTK4Cf8DDfL/g6JhnA9yDxbyUsZNh4WbqCKttiuXlDQx1oRZ7+UjW+eAnsX9z29oKcJ+ PAu7jcwlgyxBYiRU5ftcWWJ7Gn3QkdJA64JA3T9sbVorcxrAnAoGtNspn3jiHTN8oLc/mf1vtpF SH8WgE67n08AqVQJl4T/7G71P5Y8HYrdbBVJd+UUOzJb8RgBCMd3W/WkNdZM9FImibswbs4RwJ9 ngkPyQCBqpunjbuY5pfsQ1wtUUxJ5Druyp6fw+s9P9txym6MbDPlpnmO+mJ7gra3wKWIE7bBw7C sig/FQdmdFqHR4ECkueJHCdYGyFpCtOizIrXhZlBiTKaAsLXYkFWucZie8Vx6U45VbEXeXYjd4I k7qYZUOTf3jzsBKjgdbCYEBE5gtDDrrBg2GkERKcmj9YhV8bethbD2IUKuJZ1ZEvpJYRODNquYQ lEI7EeZewW18LJRyntg== X-Authority-Analysis: v=2.4 cv=eNceTXp1 c=1 sm=1 tr=0 ts=699dcb85 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=kj9zAlcOel0A:10 a=HzLeVaNsDn8A:10 a=VkNPw1HP01LnGYTKEx00:22 a=Mpw57Om8IfrbqaoTuvik:22 a=GgsMoib0sEa3-_RKJdDe:22 a=KcHrAfEX3OFbcP1ITbUA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-GUID: iyS6-Hv6LPaviWCj5CG_MQcSN9j2j3tb X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-02-24_02,2026-02-23_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 priorityscore=1501 phishscore=0 suspectscore=0 adultscore=0 bulkscore=0 lowpriorityscore=0 malwarescore=0 impostorscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2601150000 definitions=main-2602240129 On Tue, Feb 24, 2026 at 04:20:32PM +0100, Peter Zijlstra wrote: > > With PREEMPT_LAZY this turns into: > > > > 0000000000000000 : > > 0: c0 04 00 00 00 00 jgnop 0 > > 6: eb af f0 68 00 24 stmg %r10,%r15,104(%r15) > > c: b9 04 00 ef lgr %r14,%r15 > > 10: b9 04 00 b2 lgr %r11,%r2 > > 14: e3 f0 ff c8 ff 71 lay %r15,-56(%r15) > > 1a: e3 e0 f0 98 00 24 stg %r14,152(%r15) <- up to here: create stack frame > > So some of that could be elided with that asm call thunk thing we talked > about yesterday, right? Yes, with #define __preempt_schedule_notrace() \ asm volatile("brasl %%r14,preempt_schedule_notrace_thunk" : : : "cc", "memory", "r14") we would end up with: 0000000000000000 : 0: c0 04 00 00 00 00 jgnop 0 6: eb 01 03 a8 00 6a asi 936,1 c: c0 10 00 00 00 00 larl %r1,c e: R_390_PC32DBL .data..percpu+0x2 12: a7 39 00 00 lghi %r3,0 16: e3 10 33 b8 00 08 ag %r1,952(%r3) 1c: eb 22 10 00 00 f8 laa %r2,%r2,0(%r1) 22: eb ff 03 a8 00 6e alsi 936,-1 28: a7 a4 00 03 jhe 2e 2c: 07 fe br %r14 2e: e3 e0 f0 88 00 24 stg %r14,136(%r15) 34: c0 e5 00 00 00 00 brasl %r14,34 36: R_390_PC32DBL preempt_schedule_notrace_thunk+0x2 3a: e3 e0 f0 88 00 04 lg %r14,136(%r15) 40: 07 fe br %r14 The stack setup is gone, like wanted :)