Re: [Xen-devel] [RFC v2 14/15] Suppress posting interrupts when 'SN' is set

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

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Wednesday, June 10, 2015 2:49 PM
> To: Wu, Feng
> Cc: andrew.cooper3@xxxxxxxxxx; george.dunlap@xxxxxxxxxxxxx; Tian, Kevin;
> Zhang, Yang Z; xen-devel@xxxxxxxxxxxxx; keir@xxxxxxx
> Subject: Re: [RFC v2 14/15] Suppress posting interrupts when 'SN' is set
> 
> >>> On 08.05.15 at 11:07, <feng.wu@xxxxxxxxx> wrote:
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -1664,9 +1664,20 @@ static void
> __vmx_deliver_posted_interrupt(struct vcpu *v)
> >
> >  static void vmx_deliver_posted_intr(struct vcpu *v, u8 vector)
> >  {
> > +    int r, sn;
> > +
> >      if ( pi_test_and_set_pir(vector, &v->arch.hvm_vmx.pi_desc) )
> >          return;
> >
> > +    /*
> > +     * Currently, we don't support urgent interrupt, all interrupts
> > +     * are recognized as non-urgent interrupt, so we cannot send
> > +     * posted-interrupt when 'SN' is set.
> > +     */
> > +
> > +    sn = v->arch.hvm_vmx.pi_desc.sn;
> > +    r = pi_test_and_set_on(&v->arch.hvm_vmx.pi_desc);
> 
> I'm probably misunderstanding something here, but to me this looks
> like a change that would need to be done quite a bit earlier in the
> series (i.e. at this point it looks like it's fixing a bug/oversight of an
> earlier patch).

From hardware p.o.v, if 'SN' is set, the hardware doesn't send notification 
event.
vmx_deliver_posted_intr() is the software way to delivery posted-interrupts, so
we need to follow the HW's behavior. Hence we check 'SN' first, and not send
notification event if it is set.

> 
> Apart from that I'm also not understanding the synchronization
> aspect here: What if SN gets set after having been latched above,
> but before the latched value gets looked at below?

Yes, that is a question. Here is the scenario your described above, right?

        ......

        sn = v->arch.hvm_vmx.pi_desc.sn; /*sn gets 0 here*/

        v->arch.hvm_vmx.pi_desc.sn gets set by others

        else if ( !r && !sn ) /*Oops, sn cannot reflect the real 
v->arch.hvm_vmx.pi_desc.sn here*/

        ......

Maybe I need think about how to handle this.

Thanks,
Feng

> 
> Jan


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