Re: [Xen-devel] domain_pause and vcpu_pause on currently executing domain?

[George Washington Dunlap III <dunlapg@xxxxxxxxx>]

Yeah, I think I see the confusion now: one might think that 
vcpu_pause(current) would be the same as a yield() [if such a function 
actually existed].
I'm testing out alternate versions of the functions, 
vcpu_pause_delay() and domain_pause_delay(), which will explicitly is 
supposed to raise a softirq and return if vcpu == current.
We're using basically the exact same functionality, with a pause/unpause 
"counter" rather than a bit, with our execution replay system.  In our 
current implementation, there are a number of different threads of the 
replay system, each doing a small set of things: one is delivering 
interrupts, doing scheduling, etc.; but we have separate threads called 
"replay drivers" which sit between the replaying domain and the thing 
they're talking to.   All of these competeing interests want to pause and 
unpause the domain, and having just a bit isn't enough.
But I'd naturally rather use mainline xen functionality, than maintain my 
own copy, if I can.
Peace,
 -George


On Mon, 18 Jul 2005, Keir Fraser wrote:

> On 18 Jul 2005, at 21:33, George Washington Dunlap III wrote:
>
> > I just discovered the domain_pause() and vcpu_pause() functionality, and i 
> > was wondering why the functions call BUG if the target domain/vcpu is 
> > currently executing.  It seems like being able to say, "Pause the 
> > currently running domain as soon as this hypervisor event is done" would 
> > be a useful one; all that would be required is having domain_sleep_sync() 
> > raise the schedule softirq if it's the currently executing one.
> > Was there a particular reason this was implemented this way?  Or was the 
> > use I described not envisioned?
> We do have something a bit like that, for debuggers 
> (domain_pause_for_debugger). Apart from that, I'm not sure what it would be 
> useful for. Well, actually it would be useful if you could pause the current 
> domain to get it off the scheduler runqueues but still carry on your current 
> thread of execution within Xen. But Xen context switching (on x86 at least) 
> doesn't really allow that. And if you can't do that, hacks to make 
> domain_pause() at least appear to work for the currently-executing domain 
> probably aren;t very useful and would quite likely lead to confused people 
> writing subtly broken code. :-)
> Those interfaces are still open for comments though, and can change if there 
> is a strong argument to do so.
> -- Keir
+-------------------+----------------------------------------
| dunlapg@xxxxxxxxx | http://www-personal.umich.edu/~dunlapg 
+-------------------+----------------------------------------
|  Who could move a mountain, who could love their enemy?
|  Who could rejoice in pain, and turn the other cheek?
|       - Rich Mullins, "Surely God is With Us"
+------------------------------------------------------------
| Outlaw Junk Email! Support HR 1748 (www.cauce.org)

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel