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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.06.15 at 07:18, <feng.wu@xxxxxxxxx> wrote:
> @@ -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;
> +            }
> +
> +            prev.control = cmpxchg(&v->arch.hvm_vmx.pi_desc.control,
> +                                   old.control, new.control);
> +        } while ( prev.control != old.control );

This pretty clearly demonstrates that mixing bitfields and non-bitfield
mask operations makes code hard to read: How is one supposed to
see at the first glance that e.g. prev.on and
old.control & (1 << POSTED_INTR_ON) are the same thing?

Jan


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