From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id E64851A0B92 for ; Fri, 28 Nov 2014 07:50:26 +1100 (AEDT) Date: Thu, 27 Nov 2014 14:50:15 -0600 From: Segher Boessenkool To: Peter Bergner Subject: Re: [PATCH] powerpc: 32 bit getcpu VDSO function uses 64 bit instructions Message-ID: <20141127205015.GA10073@gate.crashing.org> References: <1417036288-22079-1-git-send-email-anton@samba.org> <1417041497.12952.2.camel@concordia> <1417045827.16862.32.camel@otta> <20141127160829.GA26139@gate.crashing.org> <1417110100.16862.36.camel@otta> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1417110100.16862.36.camel@otta> Cc: linuxppc-dev@lists.ozlabs.org, Anton Blanchard , paulus@samba.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Nov 27, 2014 at 11:41:40AM -0600, Peter Bergner wrote: > On Thu, 2014-11-27 at 10:08 -0600, Segher Boessenkool wrote: > > On Wed, Nov 26, 2014 at 05:50:27PM -0600, Peter Bergner wrote: > > > Nope, you don't get a SIGILL when executing 64-bit instructions in > > > 32-bit mode, so it'll happily just execute the instruction, doing > > > a full 64-bit compare. I'm guessing that the upper 32-bits of both > > > r3 and r4 contain zeros, so we're probably just getting lucky. > > > > You will get a SIGILL if you run on 32-bit hardware. > > Ha, I completely forgot about 32-bit hardware. Anyway, I looked > at the ISA, and cmpdi and cmpwi are just extended mnemonics for > cmpi, with cmpdi setting the L field to 1. Probably on 32-bit > hardware, the hardware is just ignoring the L bit being set and > doing a cmpwi for us??? Huh. Yes, maybe some implementations do that. The good news is that those then compute the correct thing ;-) Can QEMU help catch such bugs more reliably? Segher