linux-kernel - Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq

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Message-ID: <20180703170230.GI2458@hirez.programming.kicks-ass.net>
Date:   Tue, 3 Jul 2018 19:02:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>, michal.simek@...inx.com,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs

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.