Re: [PATCH] fix kvmclock bug

[Jan Kiszka <jan.kiszka@siemens.com>]

Am 27.09.2010 21:00, Zachary Amsden wrote:
> On 09/25/2010 11:54 PM, Jan Kiszka wrote:
>> That only leaves us with the likely wrong unstable declaration of the
>> TSC after resume. And that raises the question for me if KVM is actually
>> that much smarter than the Linux kernel in detecting TSC jumps. If
>> something is missing, can't we improve the kernel's detection mechanism
>> which already has suspend/resume support?
>>    
> 
> Linux must make the the conservative choice about TSC being declared
> unstable; if it is possible that it has become unstable, it is
> unstable.  Unfortunately, this bodes not well for us, as most of the
> finer points of accuracy depend on having a stable TSC.
> 
> There's a bunch of places that declare TSC unstable, and where in the
> suspend / resume cycle that happens would depend on your actual hardware.

It's absolutely clear where this happens: kvm_arch_vcpu_load. And it
seems to happen as the TSC is reset due to suspend-to-RAM.

Again: Linux recovers from this and continues to use the TSC. KVM is
more picky, so my question is if this is really required.

Jan

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