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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David.Laight@ACULAB.COM, carlos@redhat.com,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [RFC PATCH 1/2] rseq: extend struct rseq with numa node id
References: <20220131205531.17873-1-mathieu.desnoyers@efficios.com>
        <8735l3k3hu.fsf@mid.deneb.enyo.de>
        <20220201110852.GW20638@worktop.programming.kicks-ass.net>
Date:   Tue, 01 Feb 2022 13:11:54 +0100
In-Reply-To: <20220201110852.GW20638@worktop.programming.kicks-ass.net> (Peter
        Zijlstra's message of "Tue, 1 Feb 2022 12:08:52 +0100")
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* Peter Zijlstra:

> On Mon, Jan 31, 2022 at 10:19:09PM +0100, Florian Weimer wrote:
>> It can be used to implement getcpu purely in userspace, too.  I had
>> plan to hack this together with a node ID cache in TLS, which should
>> offer pretty much the same functionality (except for weird CPU
>> topology changes which alter the node ID of a previously used CPU).
>
> PowerPC does that quite a lot..

How so?  Why make it especially hard for userspace?