Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751207Ab3HFEOn (ORCPT ); Tue, 6 Aug 2013 00:14:43 -0400 Received: from mail.openrapids.net ([64.15.138.104]:33822 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750975Ab3HFEOm (ORCPT ); Tue, 6 Aug 2013 00:14:42 -0400 Date: Tue, 6 Aug 2013 00:14:37 -0400 From: Mathieu Desnoyers To: "H. Peter Anvin" Cc: Linus Torvalds , Steven Rostedt , LKML , gcc , Ingo Molnar , Thomas Gleixner , David Daney , Behan Webster , Peter Zijlstra , Herbert Xu Subject: Re: [RFC] gcc feature request: Moving blocks into sections Message-ID: <20130806041437.GA30449@Krystal> References: <51FFEC56.6040206@linux.intel.com> <1375727010.22073.110.camel@gandalf.local.home> <51FFEEEC.5060902@linux.intel.com> <1375728583.22073.118.camel@gandalf.local.home> <51FFF430.1060701@linux.intel.com> <20130805195446.GA22359@Krystal> <20130805212855.GA23044@Krystal> <52001C92.3070209@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52001C92.3070209@linux.intel.com> X-Editor: vi X-Info: http://www.efficios.com User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2508 Lines: 76 * H. Peter Anvin (hpa@linux.intel.com) wrote: > On 08/05/2013 02:28 PM, Mathieu Desnoyers wrote: > > * Linus Torvalds (torvalds@linux-foundation.org) wrote: > >> On Mon, Aug 5, 2013 at 12:54 PM, Mathieu Desnoyers > >> wrote: > >>> > >>> I remember that choosing between 2 and 5 bytes nop in the asm goto was > >>> tricky: it had something to do with the fact that gcc doesn't know the > >>> exact size of each instructions until further down within compilation > >> > >> Oh, you can't do it in the coompiler, no. But you don't need to. The > >> assembler will pick the right version if you just do "jmp target". > > > > Yep. > > > > Another thing that bothers me with Steven's approach is that decoding > > jumps generated by the compiler seems fragile IMHO. > > > > x86 decoding proposed by https://lkml.org/lkml/2012/3/8/464 : > > > > +static int make_nop_x86(void *map, size_t const offset) > > +{ > > + unsigned char *op; > > + unsigned char *nop; > > + int size; > > + > > + /* Determine which type of jmp this is 2 byte or 5. */ > > + op = map + offset; > > + switch (*op) { > > + case 0xeb: /* 2 byte */ > > + size = 2; > > + nop = ideal_nop2_x86; > > + break; > > + case 0xe9: /* 5 byte */ > > + size = 5; > > + nop = ideal_nop; > > + break; > > + default: > > + die(NULL, "Bad jump label section (bad op %x)\n", *op); > > + __builtin_unreachable(); > > + } > > > > My though is that the code above does not cover all jump encodings that > > can be generated by past, current and future x86 assemblers. > > > > For unconditional jmp that should be pretty safe barring any fundamental > changes to the instruction set, in which case we can enable it as > needed, but for extra robustness it probably should skip prefix bytes. On x86-32, some prefixes are actually meaningful. AFAIK, the 0x66 prefix is used for: E9 cw jmp rel16 relative jump, only in 32-bit Other prefixes can probably be safely skipped. Another question is whether anything prevents the assembler from generating a jump near (absolute indirect), or far jump. The code above seems to assume that we have either a short or near relative jump. Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/