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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Wu, Feng
> Sent: Wednesday, July 08, 2015 6:11 PM
> > From: Tian, Kevin
> > Sent: Wednesday, July 08, 2015 5:06 PM
> >
> > > 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
> 

Then it'd be clearer if you move the check the start of the loop, so
you can avoid two additional reads when the prev.on/sn is set. :-)

Thanks
Kevin

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