Re: [Xen-devel] [v3 08/15] Suppress posting interrupts when 'SN' is set

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

> -----Original Message-----
> From: Tian, Kevin
> Sent: Wednesday, July 08, 2015 5:06 PM
> To: Wu, Feng; xen-devel@xxxxxxxxxxxxx
> Cc: keir@xxxxxxx; jbeulich@xxxxxxxx; andrew.cooper3@xxxxxxxxxx; Zhang,
> Yang Z; george.dunlap@xxxxxxxxxxxxx
> Subject: RE: [v3 08/15] Suppress posting interrupts when 'SN' is set
> 
> > From: Wu, Feng
> > Sent: Wednesday, June 24, 2015 1:18 PM
> >
> > 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.
> >
> > Signed-off-by: Feng Wu <feng.wu@xxxxxxxxx>
> > ---
> > v3:
> > use cmpxchg to test SN/ON and set ON
> >
> >  xen/arch/x86/hvm/vmx/vmx.c | 32 ++++++++++++++++++++++++++++----
> >  1 file changed, 28 insertions(+), 4 deletions(-)
> >
> > diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> > index 0837627..b94ef6a 100644
> > --- a/xen/arch/x86/hvm/vmx/vmx.c
> > +++ b/xen/arch/x86/hvm/vmx/vmx.c
> > @@ -1686,6 +1686,8 @@ static void __vmx_deliver_posted_interrupt(struct
> vcpu *v)
> >
> >  static void vmx_deliver_posted_intr(struct vcpu *v, u8 vector)
> >  {
> > +    struct pi_desc old, new, prev;
> > +
> 
> move to 'else if'.
> 
> >      if ( pi_test_and_set_pir(vector, &v->arch.hvm_vmx.pi_desc) )
> >          return;
> >
> > @@ -1698,13 +1700,35 @@ static void vmx_deliver_posted_intr(struct vcpu
> *v, u8
> > vector)
> >           */
> >          pi_set_on(&v->arch.hvm_vmx.pi_desc);
> >      }
> > -    else if ( !pi_test_and_set_on(&v->arch.hvm_vmx.pi_desc) )
> > +    else
> >      {
> > +        prev.control = 0;
> > +
> > +        do {
> > +            old.control = v->arch.hvm_vmx.pi_desc.control &
> > +                          ~(1 << POSTED_INTR_ON | 1 <<
> POSTED_INTR_SN);
> > +            new.control = v->arch.hvm_vmx.pi_desc.control |
> > +                          1 << POSTED_INTR_ON;
> > +
> > +            /*
> > +             * 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.
> > +             * Besides that, if 'ON' is already set, we cannot set
> > +             * posted-interrupts as well.
> > +             */
> > +            if ( prev.sn || prev.on )
> > +            {
> > +                vcpu_kick(v);
> > +                return;
> > +            }
> 
> would it make more sense to move above check after cmpxchg?

My original idea is that, we only need to do the check when
prev.control != old.control, which means the cmpxchg is not
successful completed. If we add the check between cmpxchg
and while ( prev.control != old.control ), it seems the logic is
not so clear, since we don't need to check prev.sn and prev.on
when cmxchg succeeds in setting the new value.

Thanks,
Feng

> 
> > +
> > +            prev.control = cmpxchg(&v->arch.hvm_vmx.pi_desc.control,
> > +                                   old.control, new.control);
> > +        } while ( prev.control != old.control );
> > +
> >          __vmx_deliver_posted_interrupt(v);
> > -        return;
> >      }
> > -
> > -    vcpu_kick(v);
> >  }
> >
> >  static void vmx_sync_pir_to_irr(struct vcpu *v)
> > --
> > 2.1.0


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