Symmetric Multiprocessing: Difference between revisions

Some information (which may not be present on newer machines) dedicated for multiprocessing is available. First one must find the MP Floating Pointer Structure. It is aligned on a 16 byte boundary, and contains a signature at the start "_MP_" or 0x5F504D5F. The OS must search in the EBDA, the BIOS ROM space, and last kilobyte of "base memory"; the size of base memory is specified in a 2 byte value at 0x413 in kilobytes, minus 1K.

Here is what the structure looks like:

struct mp_floating_pointer_structure {

char signature[4];

// virtual wire mode is; all other bits are reserved

}

Here is what the configuration table, pointed to by the floating pointer structure looks like:

struct mp_configuration_table {

char signature[4]; // "PCMP"

uint8_t reserved;

}

After the configuration table, there are entry_count entries describing more information about the system, then after that there is an extended table. The entries are either 20 bytes to represent a processor, or 8 bytes for something else. Here are what the processor and IO APIC entries look like.

struct entry_processor {

uint8_t type; // Always 0

uint64_t reserved;

}

Here is an IO APIC entry.

struct entry_io_apic {

uint8_t type; // Always 2

uint32_t address; // The memory mapped address of the IO APIC is memory

}

For more information, see http://www.intel.com/design/pentium/datashts/24201606.pdf, chapter 4.

 

==Finding information using ACPI==

 

==AP startup==

===Startup Sequence===

The MP specification contains a standard method to start an AP, however it is not recommended to be used, as it contains very precise timings, which, if done incorrectly, can lead to several problems. Brendan offers an alternative method, which should be done for each AP '''individually'''. First send an init IPI and wait 10 milliseconds. Then send a SIPI, and poll for a flag to be set by the AP's trampoline code with a timeout of 1 millisecond. If the timeout was reached, send another SIPI, and poll for the same flag, but this time with a timeout of 1 second. If the AP managed to set the flag, the BSP should set another flag to allow the AP to continue (probably to wait for the scheduler to have a process it needs executing).

===Timing===

The easiest method for the timings is to use the PIT's mode 0. Write 0x30 to IO port 0x43 (select mode 0 for counter 0), then write your count value to 0x40, LSB first (e.g. write 0xA9 then 0x4 for a millisecond). To check if counter has finished, write 0xE2 to IO port 0x43, then read a status byte from port 0x40. If the 7th bit is set, then it has finished.

 

=== BSP Initialization Code ===

// get the BSP's Local APIC ID

__asm__ __volatile__ ("mov $1, %%eax; cpuid; shrl $24, %%ebx;": "=b"(bspid) : : );

 

}

}

 

=== AP Initialization Code ===

As the application processors start up in real mode, a little Assembly is needed to enter protected mode. Modify this example to your kernel's needs.

; this code will be relocated to 0x8000, sets up environment for calling a C function

.code16

; set up 32k stack, one for each core. It is important that all core must have its own stack

shll $15, %ebx

pushl %edi

; jump into C code (should never return)

ljmp $8, $ap_startup

// this C code can be anywhere you want it, no relocation needed

void ap_startup(int apicid) {

while(1);

}

 

==See Also==

 

===External Links===

*[http://www.osdever.net/tutorials/view/multiprocessing-support-for-hobby-oses-explained Multiprocessing Support for Hobby OSes Explained at BonaFide]