Re: Hypercall continuation and wait_event

[Keir Fraser <keir.xen@gmail.com>]

On 09/04/2012 20:18, "Ruslan Nikolaev" <nruslan_devel@yahoo.com> wrote:

> Thanks for the reply.
> 
> Since it can take arbitrarily long for an event to arrive (e.g., it is coming
> from a different guest on a user request), how do I need to handle this
> case?Does it mean that I only need to make sure that nothings get scheduled on
> this VCPU in the guest?

Nothing else *can* get scheduled on this VCPU in the guest. The VCPU will
sleep within wait_event within the hypercall context. Hence you must not
hold any hypervisor spinlocks either, for example.

> Also, it is not exactly clear to me how wait_event avoids the need for
> hypercall continuation. What about local_events_need_delivery() or
> softirq_pending()? Are they going to be handled by wait_event internally?

Your VCPU gets descheduled. Hence softirq_pending() is not your concern for
the duration that you're descheduled. And if local_event_need_delivery(),
that's too bad, they have to wait for the vcpu to wake up on the event.

 -- Keir

> Ruslan
> 
> 
> 
> 
> 
> 
> ----- Original Message -----
> From: Keir Fraser <keir.xen@gmail.com>
> To: Ruslan Nikolaev <nruslan_devel@yahoo.com>; "xen-devel@lists.xen.org"
> <xen-devel@lists.xen.org>
> Cc: 
> Sent: Monday, April 9, 2012 6:54 PM
> Subject: Re: [Xen-devel] Hypercall continuation and wait_event
> 
> On 09/04/2012 18:51, "Ruslan Nikolaev" <nruslan_devel@yahoo.com> wrote:
> 
>> Hi
>> 
>> I am curious how I can properly support hypercall continuation and
>> wait_event.
>> I have a dedicated VCPU in a domain which makes a special hypercall, and the
>> hypercall waits for certain event to arrive. I am using queues available in
>> Xen, so wait_event will be invoked in the hypercall once its ready to accept
>> events. However, my understanding that even though I have a dedicated VCPU
>> for
>> this hypercall, I still may need to support hypercall continuation properly.
>> (Is this the case?) So, my question is how exactly the need for hypercall
> 
> No it's not the case, the old hypercall_create_continuation() mechanism does
> not need to be used with wait_event().
> 
> -- Keir
> 
>> preemption may affect wait_event() and wait() operations, and where would I
>> need to do hypercall_preempt_check()?
>> 
>> Thank you!
>> Ruslan
>> 
>> 
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> 
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