MutliProcessor Specification Support in Linux 2.4

                  Randy Dunlap <rddunlap AT ieee.org>
                           2002-May-22


Linux supports MPS (MP spec.) versions 1.1 and 1.4.
Linux doesn't have full support for all of MPS version 1.4.

Experience has shown that Linux usually works best when the BIOS
is configured for MP Spec. version 1.1 if that is an option in
your system's BIOS.  I don't see why the MP Spec. version should
matter to Linux, but it would be an interesting exercise to find
out the differences as presented by BIOS tables, to determine
why Linux fails with MP Spec. version 1.4 in some cases, and to
fix Linux so that this wouldn't matter.

This document summarizes the major changes in MP spec. version 1.4
and their support status in Linux.


1.  Section 3.6.2.3, Symmetric I/O Mode:  new paragraph:

  The hardware must support a mode of operation in which the system
  can switch easily to Symmetric I/O mode from PIC or Virtual Wire
  mode. When the operating system is ready to switch to MP operation,
  it writes a 01H to the IMCR register, if that register is
  implemented, and enables I/O APIC Redirection Table entries.
  The hardware must not require any other action on the part of
  software to make the transition to Symmetric I/O mode.

Linux recognizes and supports this MP configuration mode.


2.  Section 3.6.4, Floating Point Exception Interrupt (partial text):

  For PC/AT compatibility, the bootstrap processor must support
  DOS-compatible FPU execution and exception handling while running
  in either of the PC/AT-compatible modes. This means that floating
  point error signals from the BSP must be routed to the interrupt
  request 13 signal, IRQ13, when the system is in PIC or virtual
  wire mode. While floating point error signals from an application
  processor need not be routed to IRQ13, platform designers may
  choose to connect the two. For example, connecting the floating
  point error signal from application processors to IRQ13 can be
  useful in the case of a platform that supports dynamic choice of
  BSP during boot. 

  In symmetric mode, a compliant system supports only on-chip
  floating point units, with error signaling via interrupt vector
  16. Operating systems must use interrupt vector 16 to manage
  floating point exceptions when the system is in symmetric mode.

| Linux does not use the floating point interrupt at all except in
| genuine i386 processor systems which are not SMP-capable.  [In these
| systems, if they wire the FPU exception line in the PC/AT-compatible
| way, a run-time check for #MF exception availability is performed.
| If the #MF exception is available, then Linux handles this interrupt
| if it happens.  [Maciej W. Rozycki]


3.  Section 3.6.8, Multiple I/O APIC Configurations:

Multiple I/O APICs are supported in Linux.


4.  MP Configuration Table

This table was made optional in MPS version 1.4.  If the table
isn't present, one of the default configurations should be used.
An extended section was also added to it for new table entry
types.

Linux supports the optional MP Configuration Table and uses a
default configuration if the MP Config. Table is not present.

Linux tolerates extended section table entries by skipping over
them if they are found.  Data in the extended table entries is
not used.


5.  Table 4-2:  MP Configuration Table Header Fields:

New or changed fields for MP Spec. version 1.4:

OEM Table Pointer:  supported in Linux.

Extended Table Length:  supported (tolerated, skipped) in Linux.

Extended Table Checksum:  supported (tolerated, skipped) in Linux.


6.  Table 4-10:  I/O Interrupt Entry Fields:
and Table 4-12:  Local Interrupt Entry Fields:

New or changed fields for MP Spec. version 1.4:

Source Bus IRQ:  Section D.3 specifies new PCI bus semantics
		for this field.

This field is used by Linux.


7.  Section 4.4, Extended MP Configuration Table Entries:

Entry types for System Address Space Mapping, Bus Hierarchy Descriptor,
and Compatibility Bus Address Space Modifier are defined.

Linux skips over (does not use) these extended MP Config. table
entries.  Apparently this isn't critical to any shipping systems.


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