Date: Sun, 4 Nov 2012 10:28:49 -0800 (PST) From: "Rodney W. Grimes" <freebsd@pdx.rh.CN85.ChatUSA.com> To: Juli Mallett <juli@clockworksquid.com> Cc: "freebsd-mips@FreeBSD.org" <freebsd-mips@freebsd.org> Subject: Re: CACHE_LINE_SIZE macro. Message-ID: <201211041828.qA4ISomC076058@pdx.rh.CN85.ChatUSA.com> In-Reply-To: <CACVs6=_BrwJ19CPj7OqKvV8boHfujVWqn96u3VPUmZ040JpAeQ@mail.gmail.com>
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The size of, geometry of, or even the existance of a cache should never appear in any ABI other than an ABI to query such data. There should also never be a compile time constant of a machine dependent value like this, it should be determined at boot/config time. If at all possible the use of this information should be well contained to a minimal set of code, as it SHALL change over time. Even if no MIPS CPU exists today with a 256 byte line, dont mean I won't decide to build one tomarrow. I might also decide to build one with a 2 byte line :-) What ABI is exposing anything about cache parameters or may be come dependent on such information? > Fellow FreeBSD/MIPSists, > > CACHE_LINE_SIZE is being used increasingly-much in ways which may have ABI > implications, etc. It is currentyl 2^6, whereas at least the Cavium Octeon > has cache lines that are actually 2^7 bytes in size. It would be nice to > expose the correct value to reduce false line sharing, etc., but making it > dependent on the CPU type raises ABI issues, as well as questions about how > to reliably get the right value to userland. It seems to me that > increasing it to 2^7 is the most viable approach, but I can imagine there > might be some concerns about that, so I wanted to run it past this list > first. Questions, comments, concerns? Are there MIPS CPUs with 2^8-byte > or larger cache lines that we support or will support or which are likely > coming over the horizon? > > Thanks, > Juli. > _______________________________________________ > freebsd-mips@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-mips > To unsubscribe, send any mail to "freebsd-mips-unsubscribe@freebsd.org" > -- Rod Grimes freebsd@freebsd.org
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