Re: [Qemu-devel] about the patch kvmclock Ensure proper env->tsc value for kvmclock_current_nsec calculation

[Li, Liang Z]

> On Thu, Aug 13, 2015 at 01:25:29AM +0000, Li, Liang Z wrote:
> > Hi Paolo & Marcelo,
> >
> >     Could please point out what issue the patch 317b0a6d8ba44e try to fix?  
> > I
> found in live migration the cpu_synchronize_all_states will be called twice,
> and it will take more than 1 ms sometimes. I try to do some optimization but
> lack the knowledge about the background.
> 
> What the code in 317b0a6d8ba44e requires is to retrieve the TSC value from
> the kernel.

I know 317b0a6d8ba44e is to retrieve the TSC value, but I don't understand why 
it is needed. During the live migration, the cpu_synchronize_all_states will be 
called later after stopping kvm-clock. The env->tsc will be updated, is that 
not enough? Or is there some case like call the 'stop_vm(RUN_STATE_PAUSED )' or 
' 'stop_vm (RUN_STATE_DEBUG) ', that require updating the env->tsc? By google, 
I find that your patch try to fix some issue, but I don't know what the exact 
issue.

Anyway, using cpu_synchronize_all_states to update the env->tsc is a littler 
expensive, especially in the stop can copy stage.

Liang

> Perhaps you can trace kvm_arch_get_regs and look why its taking that
> amount of time and what can be done to improve it.