Date: Sat, 04 Mar 2017 14:13:36 -0700 From: Ian Lepore <ian@freebsd.org> To: Pedro Giffuni <pfg@FreeBSD.org>, John Baldwin <jhb@freebsd.org> Cc: Slawa Olhovchenkov <slw@zxy.spb.ru>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r314669 - head/sys/i386/conf Message-ID: <1488662016.69705.21.camel@freebsd.org> In-Reply-To: <D81029FA-61CF-4648-A2A8-8570DEF28B14@FreeBSD.org> References: <201703041504.v24F4HMh023937@repo.freebsd.org> <20170304153228.GM15630@zxy.spb.ru> <ef83ffe1-6198-3468-d132-849c5d38f52d@apache.org> <2368011.hGEX4V32U5@ralph.baldwin.cx> <D81029FA-61CF-4648-A2A8-8570DEF28B14@FreeBSD.org>
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On Sat, 2017-03-04 at 15:49 -0500, Pedro Giffuni wrote: > > > > Il giorno 04 mar 2017, alle ore 14:43, John Baldwin <jhb@freebsd.or > > g> ha scritto: > > > > On Saturday, March 04, 2017 10:52:46 AM Pedro Giffuni wrote: > > > > > > > > > On 03/04/17 10:32, Slawa Olhovchenkov wrote: > > > > > > > > On Sat, Mar 04, 2017 at 03:04:17PM +0000, Pedro F. Giffuni > > > > wrote: > > > > > > > > > > > > > > Author: pfg > > > > > Date: Sat Mar 4 15:04:17 2017 > > > > > New Revision: 314669 > > > > > URL: https://svnweb.freebsd.org/changeset/base/314669 > > > > > > > > > > Log: > > > > > Drop i486 from the default i386 GENERIC kernel > > > > > configuration. > > > > > > > > > > 80486 production was stopped by Intel on September 2007. > > > > > Dropping the 486 > > > > > configuration option from the GENERIC kernel improves > > > > > performance > > > > > slightly. > > > > > > > > > > Removing I486_CPU is consistent at this time: we don't > > > > > support any > > > > > processor without a FPU and the PC-98 arch, which frequently > > > > > involved i486 > > > > > CPUs, is also gone so we don't test such platforms anymore. > > > > What is realy mean? > > > This means we don't do work-arounds that would be required for > > > raw 486. > > > Instead we will use the 586 instructions by default. > > This doesn't change that. The kernel already has runtime tests in > > place > > for new things on 486 and later via cpuid. > > > Hmm ..then I am wondering if I effectively changed anything? > > > > > > > > > > > > > > Some Via CPU is like i486 (by instruction set). > > > > > > > > CPU: VIA Ezra (800.04-MHz 686-class CPU) > > > > Origin="CentaurHauls" Id=0x678 Family=0x6 Model=0x7 Steppi > > > > ng=8 > > > > Features=0x803035<FPU,DE,TSC,MSR,MTRR,PGE,MMX> > > > > AMD Features=0x80000000<3DNow!> > > > > > > > 486 never had MMX extensions. > > > This is a 686, performance should improve ~4%. > > How did you measure the improvement? Keeping I486_CPU doesn't > > really > > do anything except remove a some #ifdef'd conditionals in > > identcpu.c > > and initcpu.c. It doesn't affect whether we use the TSC, MMX, > > etc. Those > > are all runtime checks based the CPU feature flags from cpuid. > > > The number came out from an old posting involving buildworld times, > which I can˙t find now :(. > Things seem to have changed a lot: it was surely using GCC back then, > I don˙t believe clang does much distinction about 486 at all. > > BTW, does it make sense to keep i586 in the configuration still? Both > i486 and i586 were once removed but later re-instated in r205336. > > Pedro. i586 hardware which is not also i686 is still alive and well, but I think at this point it's mostly used in specialized applications. I have no problem with removing i586 from GENERIC, but please don't remove underlying support for setting it in custom kernel configs. -- Ian
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