From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: x86: fix rdrand asm() Date: Wed, 25 Sep 2013 17:06:41 +0100 Message-ID: References: <5243218C02000078000F66E5@nat28.tlf.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta4.messagelabs.com ([85.158.143.247]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1VOrc1-0001hx-0L for xen-devel@lists.xenproject.org; Wed, 25 Sep 2013 16:06:49 +0000 Received: by mail-wi0-f177.google.com with SMTP id cb5so5819942wib.10 for ; Wed, 25 Sep 2013 09:06:47 -0700 (PDT) In-Reply-To: <5243218C02000078000F66E5@nat28.tlf.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich , xen-devel List-Id: xen-devel@lists.xenproject.org On 25/09/2013 16:46, "Jan Beulich" wrote: > Just learned the hard way that at least for non-volatile asm()s gcc > indeed does what the documentation says: It may move it across jumps > (i.e. ahead of the cpu_has() check). While the documentation claims > that this can also happen for volatile asm()s, if that was the case > we'd have many more problems in our code (and e,g, Linux would too). > > Signed-off-by: Jan Beulich > > --- a/xen/include/asm-x86/random.h > +++ b/xen/include/asm-x86/random.h > @@ -8,7 +8,7 @@ static inline unsigned int arch_get_rand > unsigned int val = 0; > > if ( cpu_has(¤t_cpu_data, X86_FEATURE_RDRAND) ) > - asm ( ".byte 0x0f,0xc7,0xf0" : "+a" (val) ); > + __asm__ __volatile__ ( ".byte 0x0f,0xc7,0xf0" : "+a" (val) ); Although not consistently applied, we use 'asm volatile' rather than '__asm__ __volatile__' generally. So we should here. Apart from that Acked-by: Keir Fraser > return val; > } > > >