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

[Alexander Graf]

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.

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