[Bug 59521] KVM linux guest reads uninitialized pvclock values before executing rdmsr MSR_KVM_WALL_CLOCK

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=59521





--- Comment #3 from Anonymous Emailer <anonymous@kernel-bugs.osdl.org>  2013-06-17 15:50:27 ---
Reply-To: pbonzini@redhat.com

Il 15/06/2013 19:17, bugzilla-daemon@bugzilla.kernel.org ha scritto:
> 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().

Considering how simple kvm_register_clock is, I think this is
preferrable if it works.  Ironically, commit 7069ed6 (x86: kvmclock:
allocate pvclock shared memory area, 2012-11-27), which introduced the
regression, is what should make this simpler fix possible.

Paolo

> 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.
>

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