Re: [Qemu-devel] [PATCH] kvmclock: Ensure time in migration never goes backward

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 06, 2014 at 09:18:27AM +0200, Alexander Graf wrote:
> 
> On 06.05.14 01:31, Marcelo Tosatti wrote:
> >On Mon, May 05, 2014 at 08:23:43PM -0300, Marcelo Tosatti wrote:
> >>Hi Alexander,
> >>
> >>On Mon, May 05, 2014 at 03:51:22PM +0200, Alexander Graf wrote:
> >>>When we migrate we ask the kernel about its current belief on what the guest
> >>>time would be.
> >>KVM_GET_CLOCK which returns the time in "struct kvm_clock_data".
> >>
> >>>However, I've seen cases where the kvmclock guest structure
> >>>indicates a time more recent than the kvm returned time.
> >This should not happen because the value returned by KVM_GET_CLOCK
> >(get_kernel_ns() + kvmclock_offset) should be relatively in sync
> >with what is seen in the guest via kvmclock read.
> >
> 
> Yes, and it isn't. Any ideas why it's not? This patch really just
> uses the guest visible kvmclock time rather than the host view of it
> on migration.

Effective frequency of TSC clock could be higher than NTP frequency. So NTP
correction would slow down the host clock.

> There is definitely something very broken on the host's side since
> it does return a smaller time than the guest exposed interface
> indicates.
> 
> 
> Alex

Can you please retrieve the values of system_timestamp, tsc_timestamp and
the host clock on the trace you have ?

Nothing forbids backwards time even without migration, in case the problem
is the difference in frequency between TSC and NTP corrected host clock
(point is, should figure out what is happening).