diff options
| -rw-r--r-- | src/cpu/x86/smm/smmrelocate.S | 19 |
1 files changed, 11 insertions, 8 deletions
diff --git a/src/cpu/x86/smm/smmrelocate.S b/src/cpu/x86/smm/smmrelocate.S index bc5b2da41b..18d668c9dd 100644 --- a/src/cpu/x86/smm/smmrelocate.S +++ b/src/cpu/x86/smm/smmrelocate.S @@ -54,13 +54,22 @@ .code16 /** - * This trampoline code relocates SMBASE to 0xa0000 - ( lapicid * 0x400 ) + * When starting up, x86 CPUs have their SMBASE set to 0x30000. However, + * this is not a good place for the SMM handler to live, so it needs to + * be relocated. + * Traditionally SMM handlers used to live in the A segment (0xa0000). + * With growing SMM handlers, more CPU cores, etc. CPU vendors started + * allowing to relocate the handler to the end of physical memory, which + * they refer to as TSEG. + * This trampoline code relocates SMBASE to base address - ( lapicid * 0x400 ) * * Why 0x400? It is a safe value to cover the save state area per CPU. On * current AMD CPUs this area is _documented_ to be 0x200 bytes. On Intel * Core 2 CPUs the _documented_ parts of the save state area is 48 bytes * bigger, effectively sizing our data structures 0x300 bytes. * + * Example (with SMM handler living at 0xa0000): + * * LAPICID SMBASE SMM Entry SAVE STATE * 0 0xa0000 0xa8000 0xafd00 * 1 0x9fc00 0xa7c00 0xaf900 @@ -88,13 +97,7 @@ * at 0xa8000-0xa8100 (example for core 0). That is not enough. * * This means we're basically limited to 16 cpu cores before - * we need to use the TSEG/HSEG for the actual SMM handler plus stack. - * When we exceed 32 cores, we also need to put SMBASE to TSEG/HSEG. - * - * If we figure out the documented values above are safe to use, - * we could pack the structure above even more, so we could use the - * scheme to pack save state areas for 63 AMD CPUs or 58 Intel CPUs - * in the ASEG. + * we need to move the SMM handler to TSEG. * * Note: Some versions of Pentium M need their SMBASE aligned to 32k. * On those the above only works for up to 2 cores. But for now we only |