https://bugzilla.kernel.org/show_bug.cgi?id=59521 --- Comment #2 from Eugene Batalov <eabatalov89@xxxxxxxxx> 2013-06-15 17:17:18 --- KVM pv_clock initialization for SMP CPU is called very early on SMP CPU boot stage: arch/x86/kernel/smpboot.c: __cpuinit start_secondary(void *unused) ... cpu_init(); x86_cpuinit.early_percpu_clock_init(); early_percpu_clock_init is set as virtual function in ./arch/x86/kernel/kvmclock.c: #ifdef CONFIG_X86_LOCAL_APIC x86_cpuinit.early_percpu_clock_init = kvm_setup_secondary_clock; #endif The problem is in cpu_init() which is called earlier. cpu_init() calls printk and possibly other stuff which can use timestamps. printk calls local_clock() to obtain a timestamp of a log message. On KVM guests call sequence usually ends up in kvm_clock_read but needed rdmsr is executed only in x86_cpuinit.early_percpu_clock_init(). I consider two approaches to fix the problem: 1. Swap cpu_init(); and x86_cpuinit.early_percpu_clock_init(); + Simple - We will get excessive restrictions on operations which allowed to be performed in early_percpu_clock_init() because percpu specific data is initialized only in cpu_init(). 2. Return 0ULL from kvm_clock_read untill it is initialized. + Simple too - Additional if statement inside kvm_clock_read (not serious even for performance paranoiacs) - Returning 0ULL looks ok because it is the same thing which kernel bootstrap CPU does on early boot stages. But I am not quite sure. Better to ask guys who maintain the needed relevant subsystem. I prefer the second way. It doesn't add complex restrictions to CPU bootup code. I'll send a patch soon which fixes the problem in the second way. I don't propagate such a logic to levels higher then KVM clocksource (pv_time_ops level for example) because of the following code: void __init kvmclock_init(void) ... 263 pv_time_ops.sched_clock = kvm_clock_read; 264 x86_platform.calibrate_tsc = kvm_get_tsc_khz; 265 x86_platform.get_wallclock = kvm_get_wallclock; 266 x86_platform.set_wallclock = kvm_set_wallclock; 267 #ifdef CONFIG_X86_LOCAL_APIC 268 x86_cpuinit.early_percpu_clock_init = 269 kvm_setup_secondary_clock; 270 #endif 271 x86_platform.save_sched_clock_state = kvm_save_sched_clock_state; 272 x86_platform.restore_sched_clock_state = kvm_restore_sched_clock_state; To propagate the logic I need to make changes both in x86_platform and pv_time_ops also I should make a similar fix for ia64 arch. It needs some subsystems refactoring to make the changes clean. Dont' think that its worth to fix the bug. Better to make a simple fix. -- Configure bugmail: https://bugzilla.kernel.org/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are watching the assignee of the bug. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
