Aside from being excessively slow, CPUID is problematic: Linux runs on a handful of CPUs that don't have CPUID. MOV to CR2 is always available, so use it instead.
Signed-off-by: Andy Lutomirski <[email protected]> --- arch/x86/include/asm/processor.h | 31 ++++++++----------------------- 1 file changed, 8 insertions(+), 23 deletions(-) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index 64fbc937d586..0388f3d85700 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -593,31 +593,16 @@ static __always_inline void cpu_relax(void) /* Stop speculative execution and prefetching of modified code. */ static inline void sync_core(void) { - int tmp; - -#ifdef CONFIG_X86_32 - /* - * Do a CPUID if available, otherwise do a jump. The jump - * can conveniently enough be the jump around CPUID. - */ - asm volatile("cmpl %2,%1\n\t" - "jl 1f\n\t" - "cpuid\n" - "1:" - : "=a" (tmp) - : "rm" (boot_cpu_data.cpuid_level), "ri" (0), "0" (1) - : "ebx", "ecx", "edx", "memory"); -#else /* - * CPUID is a barrier to speculative execution. - * Prefetched instructions are automatically - * invalidated when modified. + * MOV to CR2 is architecturally defined as a serializing + * instruction. It's nice because it works on all CPUs, it + * doesn't clobber registers, and (unlike CPUID) it won't force + * a VM exit. + * + * 0xbf172b23 is random poison just in case something ends up + * caring about this value. */ - asm volatile("cpuid" - : "=a" (tmp) - : "0" (1) - : "ebx", "ecx", "edx", "memory"); -#endif + write_cr2(0xbf172b23); } extern void select_idle_routine(const struct cpuinfo_x86 *c); -- 2.9.3
