linux-kernel - Re: [patch V2 14/37] rseq: Cache CPU ID and MM CID values

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <dd6755e4-a266-4a4d-847b-f1815c046fcf@efficios.com>
Date: Thu, 4 Sep 2025 13:21:51 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 14/37] rseq: Cache CPU ID and MM CID values

On 2025-09-02 09:48, Thomas Gleixner wrote:
> On Mon, Aug 25 2025 at 14:19, Mathieu Desnoyers wrote:
>> On 2025-08-23 12:39, Thomas Gleixner wrote:
>>> In preparation for rewriting RSEQ exit to user space handling provide
>>> storage to cache the CPU ID and MM CID values which were written to user
>>> space. That prepares for a quick check, which avoids the update when
>>> nothing changed.
>>
>> What should we do about the numa node_id field ?
>>
>> On pretty much all arch except powerpc (AFAIK) it's invariant for
>> the topology, so derived from cpu_id.
>>
>> On powerpc, we could perhaps reset the cached cpu_id to ~0U for
>> each thread to trigger an update ? Or just don't care about this ?
> 
> It's invariant on powerPC as well after the CPU was [hot]added to the
> kernel.
> 
> Otherwise any usage of cpu_to_node() would be broken on powerPC, no?

Agreed. I've added powerpc maintainers on the other leg of this thread
to validate my understanding.

Thanks,

Mathieu

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