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: Wednesday, July 01, 2015 9:26 PM
> To: Andrew Cooper
> Cc: Wu, Feng; 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 Tue, 2015-06-30 at 11:11 +0100, Andrew Cooper wrote:
> > On 24/06/15 06:18, Feng Wu wrote:
> 
> > > +/*
> > > + * Handle VT-d posted-interrupt when VCPU is blocked.
> > > + */
> > > +static void pi_wakeup_interrupt(struct cpu_user_regs *regs)
> > > +{
> > > +    struct arch_vmx_struct *vmx;
> > > +    unsigned int cpu = smp_processor_id();
> > > +
> > > +    spin_lock(&per_cpu(pi_blocked_vcpu_lock, cpu));
> > > +
> > > +    /*
> > > +     * FIXME: The length of the list depends on how many
> > > +     * vCPU is current blocked on this specific pCPU.
> > > +     * This may hurt the interrupt latency if the list
> > > +     * grows to too many entries.
> > > +     */
> > > +    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.

Thanks,
Feng

> 
> 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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