cpu_features.rst (2730B)
1============ 2CPU Features 3============ 4 5Hollis Blanchard <hollis@austin.ibm.com> 65 Jun 2002 7 8This document describes the system (including self-modifying code) used in the 9PPC Linux kernel to support a variety of PowerPC CPUs without requiring 10compile-time selection. 11 12Early in the boot process the ppc32 kernel detects the current CPU type and 13chooses a set of features accordingly. Some examples include Altivec support, 14split instruction and data caches, and if the CPU supports the DOZE and NAP 15sleep modes. 16 17Detection of the feature set is simple. A list of processors can be found in 18arch/powerpc/kernel/cputable.c. The PVR register is masked and compared with 19each value in the list. If a match is found, the cpu_features of cur_cpu_spec 20is assigned to the feature bitmask for this processor and a __setup_cpu 21function is called. 22 23C code may test 'cur_cpu_spec[smp_processor_id()]->cpu_features' for a 24particular feature bit. This is done in quite a few places, for example 25in ppc_setup_l2cr(). 26 27Implementing cpufeatures in assembly is a little more involved. There are 28several paths that are performance-critical and would suffer if an array 29index, structure dereference, and conditional branch were added. To avoid the 30performance penalty but still allow for runtime (rather than compile-time) CPU 31selection, unused code is replaced by 'nop' instructions. This nop'ing is 32based on CPU 0's capabilities, so a multi-processor system with non-identical 33processors will not work (but such a system would likely have other problems 34anyways). 35 36After detecting the processor type, the kernel patches out sections of code 37that shouldn't be used by writing nop's over it. Using cpufeatures requires 38just 2 macros (found in arch/powerpc/include/asm/cputable.h), as seen in head.S 39transfer_to_handler:: 40 41 #ifdef CONFIG_ALTIVEC 42 BEGIN_FTR_SECTION 43 mfspr r22,SPRN_VRSAVE /* if G4, save vrsave register value */ 44 stw r22,THREAD_VRSAVE(r23) 45 END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC) 46 #endif /* CONFIG_ALTIVEC */ 47 48If CPU 0 supports Altivec, the code is left untouched. If it doesn't, both 49instructions are replaced with nop's. 50 51The END_FTR_SECTION macro has two simpler variations: END_FTR_SECTION_IFSET 52and END_FTR_SECTION_IFCLR. These simply test if a flag is set (in 53cur_cpu_spec[0]->cpu_features) or is cleared, respectively. These two macros 54should be used in the majority of cases. 55 56The END_FTR_SECTION macros are implemented by storing information about this 57code in the '__ftr_fixup' ELF section. When do_cpu_ftr_fixups 58(arch/powerpc/kernel/misc.S) is invoked, it will iterate over the records in 59__ftr_fixup, and if the required feature is not present it will loop writing 60nop's from each BEGIN_FTR_SECTION to END_FTR_SECTION.