Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Mar 24, 2015 at 04:34:12PM +0100, Radim Krčmář wrote:
> 2015-03-23 20:21-0300, Marcelo Tosatti:
> > The following point:
> > 
> >     2. per-CPU pvclock time info is updated if the
> >        underlying CPU changes.
> > 
> > Is not true anymore since "KVM: x86: update pvclock area conditionally,
> > on cpu migration".
> 
> I think that the revert doesn't fix point 2.:  "KVM: x86: update pvclock
> [...]" changed the host to skip clock update on physical CPU change, but
> guest's task migration notifier isn't tied to it at all.

"per-CPU pvclock time info is updated if the underlying CPU changes"
is the same as
"always perform clock update on physical CPU change".

That was a requirement for the original patch, to drop migration
notifiers.

> (Guest can have all tasks pinned, so the revert changed nothing.)
> 
> > Add task migration notification back.
> > 
> > Problem noticed by Andy Lutomirski.
> 
> What is the problem?
> 
> Thanks.

The problem is this:

T1) guest thread1 on vcpu1.
T2) guest thread1 on vcpu2.
T3) guest thread1 on vcpu1.

Inside a pvclock read loop.

Since the writes by hypervisor of pvclock area are not ordered, 
you cannot rely on version being updated _before_ 
the rest of pvclock data.

(in the case above, "has the physical cpu changed" check, inside the
guests thread1, obviously fails).