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Date:   Tue, 3 Jul 2018 11:24:29 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Heiko Carstens <heiko.carstens@de.ibm.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <andi@firstfloor.org>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Joel Fernandes <joelaf@google.com>, michal.simek@xilinx.com,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs
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On Tue, Jul 03, 2018 at 11:17:17AM +0200, Heiko Carstens wrote:

> And to answer also your question: we don't use a regular load, since we
> would have to use 'sacf' construct surrounding the load instruction which
> would be much slower.
> We have something like that implemented for the futex atomic ops, and we
> could also implement something like that for this use case
> (e.g. get_user_atomic()), if really needed.

/me digs out the s390-PoO PDF and understands.. this is because of the
split user/kernel address space stuff.

Fair enough.