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[Bug libc/12587] sysconf(_SC_*CACHE) returns 0 for all caches on some CPUs.


http://sourceware.org/bugzilla/show_bug.cgi?id=12587

--- Comment #11 from John Haxby <john.haxby at oracle dot com> 2011-03-31 09:18:08 UTC ---
(In reply to comment #10)
> For your information, this error is the same error I reported in Debian some
> time ago:
> 
> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=609389
> 
> Debian eglibc 2.13-0exp5, which includes Ulrich's patch, fixed problem in my
> case.

This is highly surprising.  That bug is for a machine whose cpuid level is 2 so
the cpuid 4 instruction that is in both Ulrich's original patch and my
variation on it would not be used.  If Ulrich's patch fixed this then you must
be using it on some different processor.

That bug would appear to be as a result of missing entries in the
intel_02_known table which would have been added for that processor quite some
time ago.

The code re-ordering in my variation of Ulrich's fix has the advantage that the
intel_02_known table need never be updated again (assuming that it covers all
the old CPUs that have cpuid level < 4).  I know that some recent
processorswith 24 way associative caches needed an update to this table, but
those same processors would also have a cpuid level > 4 and so had we been
using cpuid 4 deterministic cache parameters then the table would not have
needed to be extended for them.  For that old Pentium M processor, though, none
of the patches referred to here would have made any difference at all.   That's
right isn't it Ulrich?

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