On Wed, Sep 23, 2015 at 5:55 PM, yangoliver <yang_oliver@xxxxxxxxxxx> wrote: > > David, > > Sorry for late reply. See my inline comments. > > > On Tue, 15 Sep 2015, David Matlack wrote: > >> 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. > In fact, I think TSC trap is the only way to make TSC sync in guest OS. > > For exmaple, user space applications could use rdtsc directly, which > may cause the problem if there is no TSC emulation. Ah, I see. KVM does not provide an option to trap and emulate RDTSC to provide a stable TSC. I do not know of any plans to change this. >> >> > >> > 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. > > Yes. I knew how to check the hardware capabilitiy. > > My major question was whether KVM support TSC sync even running over > TSC unreliable hardware. >> >> > >> > 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. > > The major problem is user application, as my application calls rstsc under > multi-thread environment. > >> > >> > 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? > > Not quite sure. We plan to run our application over KVM. > VMware ESX5.x supported TSC sync even underlying hardware TSC reliable, > so I'm just wondering KVM cases. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
