On Tue, Dec 15, 2020 at 3:35 AM Marcelo Tosatti mtosatti@redhat.com wrote:
On Fri, Dec 11, 2020 at 10:59:59PM +0100, Paolo Bonzini wrote:
On 11/12/20 22:04, Thomas Gleixner wrote:
Its 100ms off with migration, and can be reduced further (customers complained about 5 seconds but seem happy with 0.1ms).
What is 100ms? Guaranteed maximum migration time?
I suppose it's the length between the time from KVM_GET_CLOCK and KVM_GET_MSR(IA32_TSC) to KVM_SET_CLOCK and KVM_SET_MSR(IA32_TSC). But the VM is paused for much longer, the sequence for the non-live part of the migration (aka brownout) is as follows:
pause finish sending RAM receive RAM ~1 sec send paused-VM state finish receiving RAM \ receive paused-VM state ) 0.1 sec restart /The nanosecond and TSC times are sent as part of the paused-VM state at the very end of the live migration process.
So it's still true that the time advances during live migration brownout; 0.1 seconds is just the final part of the live migration process. But for _live_ migration there is no need to design things according to "people are happy if their clock is off by 0.1 seconds only".
Agree. What would be a good way to fix this?
Could you implement the Hyper-V clock interface? It's much, much simpler than the kvmclock interface. It has the downside that CLOCK_BOOTTIME won't do what you want, but I'm not really convinced that's a problem, and you could come up with a minimal extension to fix that.