Re: [Xen-devel] [v3 12/15] vmx: posted-interrupt handling when vCPU is blocked

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

> -----Original Message-----
> From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]
> Sent: Thursday, July 02, 2015 4:30 PM
> To: Wu, Feng
> Cc: Andrew Cooper; xen-devel@xxxxxxxxxxxxx; Zhang, Yang Z;
> george.dunlap@xxxxxxxxxxxxx; Tian, Kevin; keir@xxxxxxx; jbeulich@xxxxxxxx
> Subject: Re: [Xen-devel] [v3 12/15] vmx: posted-interrupt handling when vCPU
> is blocked
> 
> On Thu, 2015-07-02 at 04:27 +0000, Wu, Feng wrote:
> 
> > > > > +    list_for_each_entry(vmx, &per_cpu(pi_blocked_vcpu, cpu),
> > > > > +                        pi_blocked_vcpu_list)
> > > > > +        if ( vmx->pi_desc.on )
> > > > > +            tasklet_schedule(&vmx->pi_vcpu_wakeup_tasklet);
> > > >
> > > > There is a logical bug here.  If we have two NV's delivered to this
> > > > pcpu, we will kick the first vcpu twice.
> > > >
> > > > On finding desc.on, a kick should be scheduled, then the vcpu removed
> > > > from this list.  With desc.on set, we know for certain that another NV
> > > > will not arrive for it until it has been scheduled again and the
> > > > interrupt posted.
> > > >
> > > Yes, that seems a possible issue (and one that should indeed be
> > > avoided).
> > >
> > > I'm still unsure about the one that I raised myself but, if it is
> > > possible to have more than one vcpu in a pcpu list, with desc.on==true,
> > > then it looks to me that we kick all of them, for each notification.
> > >
> > > Added what Andrew's spotted, if there are a bunch of vcpus, queued with
> > > desc.on==ture, and a bunch of notifications arrives before the tasklet
> > > gets executed, we'll be kicking the whole bunch of them for a bunch of
> > > times! :-/
> >
> > As Andrew mentioned, removing the vCPUs with desc.on = true from the
> > list can avoid kick vCPUs for multiple times.
> >
> It avoids kicking vcpus multiple times if more than one notification
> arrives, yes.
> 
> It is, therefore, not effective in making sure that, even with only one
> notification, you only kick the interested vcpu.
> 
> This is the third time that I ask:
>  (1) whether it is possible to have more vcpus queued on one pcpu PI
>      blocked list with desc.on (I really believe it is);

I think it is, please see the following scenario:

When cpu masks the interrupts, and an external interrupt occurs for the
assigned device while the target vCPU2 is blocked, the wakeup notification
event handler has no chance to run, after a while, another wakeup
notification event for vCPU4 blocking on the same pCPU occurs,
after cpu unmakes the interrupts, wakeup notification handler
gets called. Then we get:
        vCPU2, desc.on = 1 and vCPU4, desc.on = 1
Then in the handler we need to kick both of them.

>  (2) if yes, whether it is TheRightThing(TM) to kick all of them, as
>      soon as any notification arrives, instead that putting together a
>      mechanism for kicking only a specific one.
> 
Why can't we kick all of them, 'desc.on = 1' means there is a pending
interrupt, when we meet this condition, kicking the related vCPU should
be the right thing to do.

Thanks,
Feng

> The fact that you're not answering is not so much of a big deal for
> me... I'll just keep asking! :-D
> 
> 
> Regards,
> 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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