CPU Features¶
Hollis Blanchard <hollis@austin.ibm.com>5 Jun 2002
This document describes the system (including self-modifying code) used in thePPC Linux kernel to support a variety of PowerPC CPUs without requiringcompile-time selection.
Early in the boot process the ppc32 kernel detects the current CPU type andchooses a set of features accordingly. Some examples include Altivec support,split instruction and data caches, and if the CPU supports the DOZE and NAPsleep modes.
Detection of the feature set is simple. A list of processors can be found inarch/powerpc/kernel/cputable.c. The PVR register is masked and compared witheach value in the list. If a match is found, the cpu_features of cur_cpu_specis assigned to the feature bitmask for this processor and a __setup_cpufunction is called.
C code may test ‘cur_cpu_spec[smp_processor_id()]->cpu_features’ for aparticular feature bit. This is done in quite a few places, for exampleinppc_setup_l2cr().
Implementing cpufeatures in assembly is a little more involved. There areseveral paths that are performance-critical and would suffer if an arrayindex, structure dereference, and conditional branch were added. To avoid theperformance penalty but still allow for runtime (rather than compile-time) CPUselection, unused code is replaced by ‘nop’ instructions. This nop’ing isbased on CPU 0’s capabilities, so a multi-processor system with non-identicalprocessors will not work (but such a system would likely have other problemsanyways).
After detecting the processor type, the kernel patches out sections of codethat shouldn’t be used by writing nop’s over it. Using cpufeatures requiresjust 2 macros (found in arch/powerpc/include/asm/cputable.h), as seen in head.Stransfer_to_handler:
#ifdef CONFIG_ALTIVECBEGIN_FTR_SECTION mfspr r22,SPRN_VRSAVE /* if G4, save vrsave register value */ stw r22,THREAD_VRSAVE(r23)END_FTR_SECTION_IFSET(CPU_FTR_ALTIVEC)#endif /* CONFIG_ALTIVEC */
If CPU 0 supports Altivec, the code is left untouched. If it doesn’t, bothinstructions are replaced with nop’s.
The END_FTR_SECTION macro has two simpler variations: END_FTR_SECTION_IFSETand END_FTR_SECTION_IFCLR. These simply test if a flag is set (incur_cpu_spec[0]->cpu_features) or is cleared, respectively. These two macrosshould be used in the majority of cases.
The END_FTR_SECTION macros are implemented by storing information about thiscode in the ‘__ftr_fixup’ ELF section. When do_cpu_ftr_fixups(arch/powerpc/kernel/misc.S) is invoked, it will iterate over the records in__ftr_fixup, and if the required feature is not present it will loop writingnop’s from each BEGIN_FTR_SECTION to END_FTR_SECTION.