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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.06.15 at 09:34, <feng.wu@xxxxxxxxx> wrote:
> Seems it is a little tricky here. What we need to do for this is
> like something below:
> 
> ......
> 
>     else if ( !pi_test_sn(&v->arch.hvm_vmx.pi_desc) ||
>           !pi_test_and_set_on(&v->arch.hvm_vmx.pi_desc) )
>     {
>         __vmx_deliver_posted_interrupt(v);
>         return;
>     }
> 
> ......
> 
> But how to handle this if 'SN' is set between pi_test_sn() and
> pi_test_and_set_on() ? Seems we cannot guarantee this two
> operations are atomic in software way.But thinking a bit
> more, maybe this solution is acceptable, 'SN' is only set when
> the vCPU's state is going to be runnable, which means the
> vCPU is not running in non-root, in this case, it doesn't matter
> whether we send notification event or not, as long as the
> interrupt is stored in PIR, and they will be delivered to guest
> during the next vm-entry.
> 
> Any ideas about this?

Unless the two functions don't do what their name suggests I
don't see why you need to invoke pi_test_sn() first - the purpose
of pi_test_and_set_on() after all is what its name says: Test
and set the flag atomically, returning the previous state.

Jan


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