Date: Thu, 01 Nov 2012 08:49:27 -0600 From: Ian Lepore <freebsd@damnhippie.dyndns.org> To: attilio@FreeBSD.org Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, Gleb Smirnoff <glebius@FreeBSD.org>, src-committers@FreeBSD.org Subject: Re: svn commit: r242402 - in head/sys: kern vm Message-ID: <1351781367.1120.140.camel@revolution.hippie.lan> In-Reply-To: <CAJ-FndBz=UXKANBkSrtxPUBcYyV4kh0LRm-HyxNbHEhkJd4oqQ@mail.gmail.com> References: <201210311807.q9VI7IcX000993@svn.freebsd.org> <CAJ-FndDRkBS57e9mzZoJWX5ugJ0KBGxhMSO50KB8Wm8MFudjCA@mail.gmail.com> <1351707964.1120.97.camel@revolution.hippie.lan> <CAJ-FndC7QwpNAjzQTumqTY6Sj_RszXPwc0pbHv2-pRGMqbw0ww@mail.gmail.com> <20121101100814.GB70741@FreeBSD.org> <CAJ-FndARMhgCRYwo0%2BS4tZ=At6rHJSz_tsy-OtHRHZKkxL-sig@mail.gmail.com> <1351778472.1120.117.camel@revolution.hippie.lan> <CAJ-FndCpcBTFpsdTLYoadG2P3oZFYq_viGSTNOc7YM5G66FFsg@mail.gmail.com> <CAJ-FndCG6WZ0Rwm9HcJhy4K32%2BC3vai2mQim_q7%2BMd427L6LbQ@mail.gmail.com> <1351780635.1120.135.camel@revolution.hippie.lan> <CAJ-FndBz=UXKANBkSrtxPUBcYyV4kh0LRm-HyxNbHEhkJd4oqQ@mail.gmail.com>
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On Thu, 2012-11-01 at 14:43 +0000, Attilio Rao wrote: > On 11/1/12, Ian Lepore <freebsd@damnhippie.dyndns.org> wrote: > > On Thu, 2012-11-01 at 14:07 +0000, Attilio Rao wrote: > >> On Thu, Nov 1, 2012 at 2:05 PM, Attilio Rao <attilio@freebsd.org> wrote: > >> > On Thu, Nov 1, 2012 at 2:01 PM, Ian Lepore > >> > <freebsd@damnhippie.dyndns.org> wrote: > >> >> On Thu, 2012-11-01 at 10:42 +0000, Attilio Rao wrote: > >> >>> On 11/1/12, Gleb Smirnoff <glebius@freebsd.org> wrote: > >> >>> > On Wed, Oct 31, 2012 at 06:33:51PM +0000, Attilio Rao wrote: > >> >>> > A> > Doesn't this padding to cache line size only help x86 > >> >>> > processors in an > >> >>> > A> > SMP kernel? I was expecting to see some #ifdef SMP so that we > >> >>> > don't > >> >>> > pay > >> >>> > A> > a big price for no gain in small-memory ARM systems and such. > >> >>> > But > >> >>> > maybe > >> >>> > A> > I'm misunderstanding the reason for the padding. > >> >>> > A> > >> >>> > A> I didn't want to do this because this would be meaning that SMP > >> >>> > option > >> >>> > A> may become a completely killer for modules/kernel ABI > >> >>> > compatibility. > >> >>> > > >> >>> > Do we support loading non-SMP modules on SMP kernel and vice versa? > >> >>> > >> >>> Actually that's my point, we do. > >> >>> > >> >>> Attilio > >> >>> > >> >>> > >> >> > >> >> Well we've got other similar problems lurking then. What about a > >> >> module > >> >> compiled on an arm system that had #define CACHE_LINE_SIZE 32 and then > >> >> it gets run on a different arm system whose kernel is compiled with > >> >> #define CACHE_LINE_SIZE 64? > >> > > >> > That should not happen. Is that a real case where you build a module > >> > for an ARM family and want to run against a kernel compiled for > >> > another? > >> > >> Besides that, the ARM CACHE_LINE_SIZE is defined in the shared headers > >> so there is no way this can be a problem. > > > > I've been under the impression that in the ARM and MIPS worlds, the > > cache line size can change from one family/series of chips to another, > > just as support for SMP can change from one family to another. If I'm > > not mistaken in that assumption, then there can't be something like a > > generic arm module that will run on any arm kernel regardless of how the > > kernel was built, not if compile-time constants get cooked into the > > binaries in a way that affects the ABI/KBI. > > I'm far from being an ARM expert so I trust what you say. > This only means you cannot build a module for a family and expect to > retain ABI compatibility among all the ARM families. If cache-lines > are different I don't think there is much we can do, which has nothing > to do with pad-align locking. > I do a lot of work with armv4 and recently v5 chips, but nothing with the v6/v7 stuff yet, so I'm not really an expert on these issues either. I've heard some talk from the folks working on arm v6/v7 support about things like unified kernels and an arm GENERIC kernel config, but I'm pretty hazy myself on how that vision is shaping up. -- Ian
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