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

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

> -----Original Message-----
> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Thursday, July 02, 2015 6:30 PM
> To: Dario Faggioli; Wu, Feng
> Cc: 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 02/07/15 09:30, Dario Faggioli wrote:
> > 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);
> >  (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.
> 
> We will receive one NV for every time the hardware managed to
> successfully set desc.on
> 
> If multiple stack up and we proactively drain the list, we will
> subsequently search the list to completion for all remaining NV's, due
> to finding no appropriate entries.
> 
> I can't currently decide whether this will be quicker or slower overall,
> or (most likely) it will even out to equal in the general case.

What do you mean by "general case"?

Thanks,
Feng

> 
> ~Andrew
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