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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.06.15 at 11:40, <feng.wu@xxxxxxxxx> wrote:

> 
>> -----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.

Right, that's what I understood; my question wasn't "why", but
"doesn't this need to be done earlier in the series".

>> 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*/
> 
>       ......

Yes.

> Maybe I need think about how to handle this.

Apparently, unless you can find a reason why this is not a problem.

Jan


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