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

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





--- Comment #4 from Eugene Batalov <eabatalov89@gmail.com>  2013-06-17 21:29:08 ---
(In reply to comment #3)
> 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

Understood your point. I'll test this fix and report the results.

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