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

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-03-24 19:59-0300, Marcelo Tosatti:
> 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).

Ah, thanks! so the "KVM: x86: update pvclock area conditionally [...]"
has nothing to do with it -- that really confused me.