From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH] x86: Fix build following c/s 623c720f "x86: use CLFLUSHOPT when available" Date: Fri, 12 Feb 2016 09:51:10 +0000 Message-ID: <56BDAB0E.3050208@citrix.com> References: <1455218743-12751-1-git-send-email-andrew.cooper3@citrix.com> <56BDA49A02000078000D146C@prv-mh.provo.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <56BDA49A02000078000D146C@prv-mh.provo.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 Cc: Xen-devel List-Id: xen-devel@lists.xenproject.org On 12/02/16 08:23, Jan Beulich wrote: >>>> On 11.02.16 at 20:25, wrote: >> CentOS 7 gets into trouble when compiling Xen citing: >> >> flushtlb.c: Assembler messages: >> flushtlb.c:149: Error: value of 256 too large for field of 1 bytes at 1 >> >> The line number is wrong, and the error message not helpful. It turns out >> that the intermediate generated assembly was >> >> # 139 "arch/x86/flushtlb.c" 1 >> 661: >> rex clflush (%r15) >> 662: >> .pushsection .altinstructions,"a" >> >> and it was having trouble combining the explicit REX prefix with the REX.B >> required for the use of %r15. > What gas version is this? I just checked with 2.20, which has no > problem combining an explicit with a generated REX prefix. bash-4.2$ as --version GNU assembler version 2.23.52.0.1-30.el7_1.2 20130226 > Or > wait, no, your description of the issue is wrong: It actually is the > folding of the two REX prefixes which causes the problem, This is what I said. What do you think I meant with "trouble combining the" ? > since > that results in the replacement instruction being one byte longer > than the to be replaced one. But that is still the case with an explicit %ds override. The assembler still has to insert an extra byte somehow. > >> Follow what Linux does and use a redundant %ds prefix instead, for a final >> generated instruction of `3e 41 0f ae 3f` >> >> While modifying this line, fix the indentation which was out by one space. >> >> Signed-off-by: Andrew Cooper > I think before committing I'll extend your patch by introducing > and using Linux'es NOP_DS_PREFIX. Ok. ~Andrew