Date: Wed, 3 Jan 2018 10:41:41 +0100 From: Maurizio Vairani <maurizio1018@gmail.com> To: blubee blubeeme <gurenchan@gmail.com> Cc: Ian Lepore <ian@freebsd.org>, Konstantin Belousov <kostikbel@gmail.com>, David Chisnall <theraven@freebsd.org>, Adrian Chadd <adrian.chadd@gmail.com>, FreeBSD current <freebsd-current@freebsd.org> Subject: Re: Programmatically cache line Message-ID: <CAN0zgYX0EVU9=iChHxf-cXghBRc_6S=F6MSzwQdMbyBnLrxPuw@mail.gmail.com> In-Reply-To: <CALM2mEnA7GH1fuvgj%2BJLGYcEkwZQkZCvW%2Bu-sFV79N3hjMUQdg@mail.gmail.com> References: <CALM2mEmWYz5nyqvxMJwMWoFOXnDTvWFrEug7UUha6xe7Um6ODw@mail.gmail.com> <20171230082812.GL1684@kib.kiev.ua> <CAJ-VmomxGJsn8eOtWoqevdW-spUPgcSGKEc7eR4xuXLP-E1XRA@mail.gmail.com> <08038E36-9679-4286-9083-FCEDD637ADCC@FreeBSD.org> <20180101103655.GF1684@kib.kiev.ua> <1514823989.12000.30.camel@freebsd.org> <CALM2mEnA7GH1fuvgj%2BJLGYcEkwZQkZCvW%2Bu-sFV79N3hjMUQdg@mail.gmail.com>
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2018-01-02 2:27 GMT+01:00 blubee blubeeme <gurenchan@gmail.com>: > On Tue, Jan 2, 2018 at 12:26 AM, Ian Lepore <ian@freebsd.org> wrote: > > > On Mon, 2018-01-01 at 12:36 +0200, Konstantin Belousov wrote: > > > On Mon, Jan 01, 2018 at 06:52:37AM +0000, David Chisnall wrote: > > > > > > > > On 1 Jan 2018, at 05:09, Adrian Chadd <adrian.chadd@gmail.com> > wrote: > > > > > > > > > > > > > > > On 30 December 2017 at 00:28, Konstantin Belousov wrote: > > > > > > > > > > > > On Sat, Dec 30, 2017 at 07:50:19AM +0000, blubee blubeeme wrote: > > > > > > > > > > > > > > Is there some way to programmatically get the CPU cache line > > sizes on > > > > > > > FreeBSD? > > > > > > There are, all of them are MD. > > > > > > > > > > > > On x86, the CPUID instruction leaf 0x1 returns the information in > > > > > > %ebx register. > > > > > Hm, weird. Why don't we extend sysctl to include this info? > > > For the same reason we do not provide a sysctl to add two integers. > > > > > > > > > > > > > > > It would be nice to expose this kind of information via VDSO or > > similar. There are a lot of similar bits of info that people want to use > > for ifunc and, SVE is going to have a bunch of similar requirements. > > > > > > > Is VDSO a new trendy word ? > > > > > > ifunc resolvers in usermode on FreeBSD/x86 get four arguments which > > > are essentially cpu_features / cpu_features2 / cpu_stdext_features / > > > cpu_stdext_features2. I suspect that only FreeBSD/x86 arches have the > > > ifunc support, in rtld and coming shortly in kernel. > > > > > > Recently HW_CAP/HW_CAP2 were added to the ELF auxv, and elf_aux_info(3) > > > interface exported from libc. > > > > > > ARM* did not implemented yet the ifunc stubs in rtld. I believe this is > > > considered a low priority because there is no ready to use toolchain > > > which allow to utilize ifuncs on FreeBSD, except if you use recent bfd > > > ld externally. > > > > Linux exports this info using getauxval(). I think we should support > > getauxval() and as many of the AT_* values that linux defines as makes > > sense for us to do. > > > > I think it was a mistake to give our version of the function a > > different name and different semantics, but this is something that > > affects mainly ports, and I don't yet have enough info to make the case > > that being linux-compatible will ease porting rather than complicate it > > (in some cases, patches will be needed either way). > > > > -- Ian > > > FreeBSD implements hardware specific atomic instructions [man atomic] or > look at: #include <machine/atomic.h> > > but implementing something that returns size of cache lines is somehow out > of the question? > > If you're working with atomic data structures and want to ensure there's no > false sharing the > simplest method I know is to put some padding that's sizeof(cache_line) - > sizeof(data_members) > so that you can try to get them to live on different cache line. > > Do we have to go in and write inline assembly to grab the size of the cache > line or wouldn't it > be simpler to have atomic.h return this info? > > You can use CPUCTL(4).
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