Date: Mon, 14 Jan 2008 16:45:15 +0000 (UTC) From: Csaba Henk <csaba-ml@creo.hu> To: freebsd-current@freebsd.org Subject: Re: RFC: Adding a hw.features[2] sysctl Message-ID: <slrnfon4at.i6j.csaba-ml@beastie.creo.hu> References: <1200197787.67286.13.camel@shumai.marcuscom.com> <20080113182457.GN929@server.vk2pj.dyndns.org> <a2b6592c0801131721w25afae5bg3dcf6a90c1a3d2b7@mail.gmail.com> <200801141254.20400.doconnor@gsoft.com.au> <a2b6592c0801131838jcde3634le6087d2f784adcbc@mail.gmail.com> <478AE741.1000105@comcast.net> <a2b6592c0801140139v42bb6ab2s667ebceb9ba3ab16@mail.gmail.com> <slrnfomt3j.i6j.csaba-ml@beastie.creo.hu> <a2b6592c0801140747r2a7ec1e5q7cd858065399ae65@mail.gmail.com>
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On 2008-01-14, Igor Mozolevsky <igor@hybrid-lab.co.uk> wrote:
> On 14/01/2008, Csaba Henk <csaba-ml@creo.hu> wrote:
>> Hm, I just fail to see the how the ioctl interface is different from
>> the sysctl interface in terms of semantic capabilites.
>
> You need to *define* the output of a sysctl, you don't have to produce
> any output in ioctl, just a boolean reply or a mask that can be
> processed with #define macros... I honestly don't see how all of that
> can be abstracted away in a MIB given that there is a number of
> Intel|AMD|Whoever feature/feature1, who knows when feature2 will be
> needed...
I don't pretend that I'm an expert of the topic discussed, I just don't
see any essential difference (in terms of convenience or expressive
power) between these:
################################################
/* your code sample */
cpu_features_t mask;
fd = open( "/dev/cpuinfo", O_RDONLY );
ioctl( fd, SIOCGCPUFEATURES, (caddr_t)&mask );
close( fd );
################################################
################################################
cpu_features_t mask;
size_t len = sizeof(mask);
sysctlbyname("hw.features", &mask, &len, NULL, 0);
################################################
(and then the macro based processing of the mask
can be done exactly the same way).
> If it can be done reasonably in a MIB, I won't say a word, but
> nobody's proposed any data representation for a (or a number of)
> MIB(s) yet...
>
> What's the overhead of sysctl vs ioctl?
Well, looking at the above code examples, the ioctl based implementation
costs three syscalls, the sysctl based one costs only two.
Csaba
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