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Date:   Tue, 3 Jul 2018 19:02:30 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andi Kleen <andi@firstfloor.org>
Cc:     Heiko Carstens <heiko.carstens@de.ibm.com>,
        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>, 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 09:40:48AM -0700, Andi Kleen wrote:
> > 
> > So I think you're good... But yes, you raise an interresting point.
> 
> So it sounds like architectures that don't have an instruction atomic u64
> *_user need to disable interrupts during the access, and somehow handle that
> case when a page fault happens?

So for 32bit, as Linus already said, a split store is _fine_, because
the top word is always going to be 0 anyway.

So all we really need is native word sized loads / stores. s390 is just
a little weird here (it wouldn't be s390 if it wasn't I suppose) for not
actually using regular loads / stores because of the split address space
stuff.