Re: Questions about KVM TSC trapping

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On Tue, Sep 15, 2015 at 12:04 AM, Oliver Yang <yang_oliver@xxxxxxxxxxx> wrote:
> Hi Guys,
>
> I found below patch for KVM TSC trapping / migration support,
>
> https://lkml.org/lkml/2011/1/6/90
>
> It seemed the patch were not merged in Linux mainline.
>
> So I have 3 questions here,
>
> 1.  Can KVM support TSC trapping today? If not, what is the plan?

Not without a patch. Did you want to trap RDTSC? RDTSC works without
trapping thanks to hardware support.

>
> 2. What is the solution if my SMP Linux guest OS doesn't have reliable
> TSC?

If you are seeing an unreliable TSC in your guest, maybe your host
hardware does not support a synchronized TSC across CPUs. I can't
recall how to check for this. There might be a flag in your host's
/proc/cpuinfo.

>
> Because the no TSC trapping support, will kvmclock driver handle all
> TSC sync issues?
>
> 3. What if my Linux guest doesn't have kvmclock driver?

The guest will use a different clock source (e.g. acpi-pm). Note the
RDTSC[P] instruction will still work just fine.

>
> Does that mean I shouldn't run TSC sensitive application in my guest
> OS?
>
> BTW, my application is written with lots of rdtsc instructions, and
> which performs well in VMware guest.

Does it not work well in KVM?
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