Re: [Xen-devel] [PATCH v8 15/17] vmx: VT-d posted-interrupt core logic handling

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Tuesday, October 27, 2015 5:52 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Dario Faggioli
> <dario.faggioli@xxxxxxxxxx>; GeorgeDunlap <george.dunlap@xxxxxxxxxxxxx>;
> Tian, Kevin <kevin.tian@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxx; Keir Fraser
> <keir@xxxxxxx>
> Subject: RE: [Xen-devel] [PATCH v8 15/17] vmx: VT-d posted-interrupt core 
> logic
> handling
> 
> >>> On 27.10.15 at 06:19, <feng.wu@xxxxxxxxx> wrote:
> >> From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]
> >> Sent: Monday, October 26, 2015 10:40 PM
> >> In any case, it would have been nice, given the situation, if you'd have 
> >> put a
> few
> >> words about, e.g., which solution you ended up implementing and why,
> either in
> >> the cover or here (e.g., in the '---' area).
> >
> > Thanks for the suggestion. As I mentioned before, updating the PI descriptor
> > needs
> > to be atomic, I think it should be a little expensive. So doing it every
> > VM-exit seems
> > not a good idea to me.
> 
> You could check whether any update is needed before doing the
> (atomic) cmpxchg16b.

I am not sure why you mentioned (atomic) cmpxchg16b here, for atomically
Update PI descriptor, we don't need use cmpxchg16b, right?

In my understanding, It seems there were two suggestions about this:

#1: Set 'SN' on each VM-Exit and clear 'SN' on each VM-Entry, so we don't
need to care about the status of 'SN' in root mode. This is what I mean "not
a good idea", since we add an atomic operation on each VM-Exit/VM-Entry.

#2: For the blocking cancel case, let's take the following code for example:

void vcpu_block(void)
{
    struct vcpu *v = current;

    set_bit(_VPF_blocked, &v->pause_flags);

    arch_vcpu_block(v);

    /* Check for events /after/ blocking: avoids wakeup waiting race. */
    if ( local_events_need_delivery() )
    {
        clear_bit(_VPF_blocked, &v->pause_flags);
        /* arch_vcpu_block_cancel(v); */ 
    }
    else
    {
        TRACE_2D(TRC_SCHED_BLOCK, v->domain->domain_id, v->vcpu_id);
        raise_softirq(SCHEDULE_SOFTIRQ);
    }
}

We still call arch_vcpu_block() in vcpu_block(), but we don't need to
call arch_vcpu_block_cancel(v) when local_events_need_delivery()
returns true. Then before VM-Entry, we can check if 'NV' field is
' pi_wakeup_vector', if it is, we change the PI status and remove
it from the blocking list.

Jan, if #2 is what you mean, I think it worth a try. If I don't understand
your comments correctly, could you please elaborate it more? Thanks
a lot!

Thanks,
Feng

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