From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs
Date: Tue, 3 Jul 2018 11:24:29 +0200
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To: Heiko Carstens <heiko.carstens@de.ibm.com>
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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.