Re: [Xen-devel] [PATCH 02/18] xen/arm: Implement PSCI system suspend call (virtual interface)

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Thu, 2018-11-15 at 11:38 +0000, Julien Grall wrote:
> On 11/15/18 11:10 AM, Mirela Simonovic wrote:
> I don't think we are discussing the same thing. The discussion was 
> around other vCPUs, not the vCPU calling SYSTEM_SUSPEND.
> 
> Most likely in the future, we would want to allow the toolstack to 
> request resuming the domain. This can be considered as an event.
> 
> > Given the assumption, my understanding is that Xen itself will not
> > unblock vCPU, except due to an interrupt targeted to the guest.
> > Am I missing something? An example would be appreciated.
> 
> At least on Arm, the current semantics of vcpu_block/vcpu_unblock is
> to 
> block until you receive an events.
> 
> I don't much want to restrict the definition of events to only 
> interrupts.  To clarify my point, if you want to wake-up for any
> events 
> then fine. 
>
So, I certainly lack deep knowledge of PSCI, as well as other aspects
of how this suspend/resume logic will work, but just to clarify on
this.

vcpu_unblock() may indeed be called from a number of places, but it
actually wakes-up the vcpu _iff_ the vcpu is runnable. If it is not
--e.g., because pause_count is not zero, or any vcpu or domain
pause_flags are set-- the vcpu stays blocked (check the implementation
of vcpu_wake() and of vcpu_runnable()).

So, it looks to me that what you want is to be sure that when an event
arrives, but the vcpus need to remain suspended, vcpu_runnable()
returns false for them. OTOH, when the events that you want to wake
them up arrives, you want it to return true.

Whether that is better done by using pause_count (plus some other flag,
as Stefano is saying) or with either an existing or new vcpu or domain
pause flag, I don't know, but it looks like it could work to me.

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Software Engineer @ SUSE https://www.suse.com/

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