Re: [PATCH] Remove a set operation for VCPU_KICK_SOFTIRQ when post interrupt to vm.

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Fri, 2015-09-18 at 11:50 +0000, Zhang, Yang Z wrote:
> Andrew Cooper wrote on 2015-09-18:
> > On 18/09/15 12:40, Zhang, Yang Z wrote:

> > >  But it only happens before do vmentry. Is it possible that
> > VCPU been preempted at any time when running in hypervisor mode?
> > 
> > All vcpus may be preempted at any time by the Xen scheduler.
> > 
> > However, paths through the hypervisor are synchronous, so a vcpu
> > will
> > notice that it has been preempted on the return-to-guest path, and
> > a
> > context switch will occur as appropriate.
> 
> So the context switch only occurs on the return-to-guest patch or if
> the VCPU call schedule() by itself. It seems a little different from
> linux kernel which possible to preempt a task at any point.
> 
It is, indeed. Xen does not 'preempt itself', like Linux does, and you
also don't call schedule() directly (like Linux does).

In fact, any time you want to reschedule, you need to raise
SCHEDULE_SOFTIRQ, which is then checked and serviced in do_softirq()
(and, for instance, not in process_pending_softirq(), specifically to
avoid preemptions!).

Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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