On 06/07/2016 15:01, Andy Lutomirski wrote: > On Jul 5, 2016 1:36 PM, "Paolo Bonzini" <pbonzini@xxxxxxxxxx> wrote: >> >> Bad things happen if a guest using the TSC deadline timer is migrated. >> The guest doesn't re-calibrate the TSC after migration, and the >> TSC frequency can and will change unless your processor supports TSC >> scaling (on Intel this is only Skylake) or your data center is perfectly >> homogeneous. >> >> The solution in this patch is to skip tsc_khz, and instead derive the >> frequency from kvmclock's (mult, shift) pair. Because kvmclock >> parameters convert from tsc to nanoseconds, this needs a division >> but that's the least of our problems when the TSC_DEADLINE_MSR write >> costs 2000 clock cycles. Luckily tsc_khz is really used by very little >> outside the tsc clocksource (which kvmclock replaces) and the TSC >> deadline timer. > > I'm wondering if it would be more straightforward to blacklist the > APIC timer entirely (on KVM guests that have the TSC deadline > capability) and to instead offer a paravirt clockevent that still uses > the deadline timer under the hood. After all, these systems should be > mostly quirk-free, so I imagine that the whole implementation would > only be a few lines of code. Very good idea! Paolo -- 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
