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
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