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Date: Wed, 13 Feb 2002 12:42:03 +0100 (CET)
From: Martin Wilck <Martin.Wilck@fujitsu-siemens.com>
To: Linus Torvalds <torvalds@transmeta.com>
cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Martin Wilck <Martin.Wilck@fujitsu-siemens.com>,
        Linux Kernel mailing list <linux-kernel@vger.kernel.org>,
        Marcelo Tosatti <marcelo@conectiva.com.br>
Subject: SMT, again (was: Re: [PATCH]: Fix MTRR handling on HT CPUs)
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I just found out that Intel specifies that on SMT-enabled
("Jackson") systems the BIOS MP tables list only the physical CPUs.
Logical CPUs will only be available through the ACPI tables.

See http://www.intel.com/technology/hyperthread/platform_nexgen/,
  in particular sld014.htm, sld021.htm.

This will basically obsolete the patch we were discussing, if BIOS
manufacturers comply to that spec, because linux will only see the
2 physical CPUs. (The problem we discovered was caused by our BIOS not
complying to the spec).

Of course, it would also mean that Linux will only run on ~70% of the CPU
power that Win2k/XP systems will have available on such systems.

All my attempts to get ACPI running on our SMT-enabled system have failed
so far (I'm working on a bug report on that for linux-acpi).

A possible workaround would be the "processor affinity algorithm"
sketched in sld021.htm, but it may be unreliable because it overrides
BIOS settings (BIOS-diabled CPUs) that are available only through ACPI.

Sorry to bother if you knew this already.

Martin

-- 
Martin Wilck                Phone: +49 5251 8 15113
Fujitsu Siemens Computers   Fax:   +49 5251 8 20409
Heinz-Nixdorf-Ring 1	    mailto:Martin.Wilck@Fujitsu-Siemens.com
D-33106 Paderborn           http://www.fujitsu-siemens.com/primergy





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