RE: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Avi Kivity [mailto:avi@redhat.com]
> 
> On 10/29/2009 06:15 PM, Dan Magenheimer wrote:
> > On a related note, though some topic drift, many of
> > the problems that occur in virtualization due to migration
> > could be better addressed if Linux had an architected
> > interface to allow it to be signaled if a migration
> > occurred, and if Linux could signal applications of
> > the same.  I don't have any cycles (pun intended) to
> > think about this right now, but if anyone else starts
> > looking at it, I'd love to be cc'ed.   
> 
> IMO that's not a good direction.  The hypervisor should not depend on 
> the guest for migration (the guest may be broken, or 
> malicious, or being 
> debugged, or slow).  So the notification must be asynchronous, which 
> means that it will only be delivered to applications after 
> migration has 
> completed.

I definitely agree that the hypervisor can't wait for a guest
to respond.

You've likely thought through this a lot more than I have,
but I was thinking that if the kernel received the notification
as some form of interrupt, it could determine immediately
if any running threads had registered for "SIG_MIGRATE"
and deliver the signal synchronously.

> Instead of a "migration has occured, run for the hills" signal we're 
> better of finding out why applications want to know about 
> this event and 
> addressing specific needs.

Perhaps.  It certainly isn't warranted for this one
special case of timestamp handling.  But I'll bet 5-10 years
from now, after we've handled a few special cases, we'll
wish that we would have handled it more generically.

Dan