Date: Mon, 3 Jun 2013 22:42:39 -0600 From: Warner Losh <imp@bsdimp.com> To: Patrick Kelsey <kelsey@ieee.org> Cc: Juli Mallett <jmallett@freebsd.org>, Ed Schouten <ed@80386.nl>, Adrian Chadd <adrian@freebsd.org>, "freebsd-mips@FreeBSD.org" <freebsd-mips@freebsd.org>, FreeBSD-arch <freebsd-arch@freebsd.org> Subject: Re: Kernelspace C11 atomics for MIPS Message-ID: <05C98B6B-1531-4E09-80D7-4F3B1A88FF01@bsdimp.com> In-Reply-To: <CAD44qMV2indeyD-OsJsyR_yp-HowJOrKG6Znx3zHkSKzt7xQzA@mail.gmail.com> References: <CAJOYFBD502MYbkVR2hnVDTYWOvOUr15=OPyjotNvv%2BZ09vQ1OQ@mail.gmail.com> <D02AF210-5129-40AB-9481-3F0A44575E98@bsdimp.com> <CAJ-Vmo=vNbT9majPCZ8ugzPsNzh46DTD4mMDX-cuxx9Og91ptw@mail.gmail.com> <CACVs6=_X5vOfR%2BQOgvz6P-j3jUoNoK9hCFvz80fGRL3-PgBf5g@mail.gmail.com> <CAJ-Vmo=5%2BmWk4EWBuTdpF6vKx-%2BK=g1euJvZuRDF%2BvFkJNZZ4A@mail.gmail.com> <CAD44qMUUQ6_FDiTknK1rcW=rs7Mz96X35QgnRxH%2Br5=ZqGRbAw@mail.gmail.com> <CAD44qMV2indeyD-OsJsyR_yp-HowJOrKG6Znx3zHkSKzt7xQzA@mail.gmail.com>
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On Jun 3, 2013, at 10:15 PM, Patrick Kelsey wrote: > On Tue, Jun 4, 2013 at 12:08 AM, Patrick Kelsey <kelsey@ieee.org> = wrote: >> On Mon, Jun 3, 2013 at 11:57 PM, Adrian Chadd <adrian@freebsd.org> = wrote: >>> On 3 June 2013 20:55, Juli Mallett <jmallett@freebsd.org> wrote: >>>=20 >>>> To drain the pipeline on certain deficient (and mostly older) CPUs = by way of >>>> guesswork and a little vague magic. Most CPUs we support, I would = guess, do >>>> not need this, and it continues to exist solely for hysterical = reasons. >>>=20 >>> How can I turn it off for my compiles? >>>=20 >>>> I've certainly gotten rid of them and some other cargo cult = synchronization >>>> on Octeon for testing and had it survive under considerable load, = and >>>> occasionally with some slight speedups (for some more commonly-used = or >>>> slower things than Just a Bunch Of NOPs.) >>>=20 >>> Right. Well, since it's happening on every inlined lock, it's a bit = silly. >>>=20 >>>> The trouble is that proving they aren't necessary requires being = rigorous >>>> and careful in understanding documentation and errata, and FUD = about their >>>> possible necessity is somewhat-intimidating. It's not an easy kind = of >>>> corruption/unreliability/etc., to prove the lack of empirically. >>>=20 >>> I've checked the diassembly from gcc-4.mumble on linux; it doesn't >>> include NOPs like this as far as I can tell. >>>=20 >>=20 >> The sync + 8 nops is coming from the definition of mips_sync() in >> sys/mips/include/atomic.h. They came from the old mips2 branch, which may have been from the = Juniper code merge, or maybe not. >> I agree with Juli that it appears to be a manual pipeline-flush >> holdover from earlier days - I'm guessing there's 8 nops because the >> R4000/4400 had both the sync instruction and an 8-stage pipeline. = I'm >> further guessing this was an attempt at providing stronger ordering >> semantics than the sync instruction itself for the following >> mb()/wmb()/rmb() definitions that use it, as the sync instruction >> definition doesn't restrict execution of the before/after = loads/stores >> with respect to the sync instruction itself. >=20 > Forgot to emphasize that this particular bit of old-school > nop-counting is either pointless or a latent hazard - 8 does not cover > the deepest MIPS pipeline around, then there's superscalar issue to > consider - so I think it's either unnecessary or insufficient. So > far, that's all criticism and no solution :/ Yes, there's new nops for these situations starting in mips32r2 and = mips64r2 ISAs. I think that this originated in the mips-jnpr merge, but can't find the = old branches anywhere... Warner
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