Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Fri, 2015-09-18 at 00:27 -0600, Jan Beulich wrote:
> > George Dunlap <george.dunlap@xxxxxxxxxx> 09/17/15 4:30 PM >>>The

> > > > vcpu_block()
>  >set(_VPF_blocked)
>  >v->arch.block()
>   >- Add v to pcpu.pi_blocked_vcpu
>   >- NV => pi_wakeup_vector
>  >local_events_need_delivery()
>    >hvm_vcpu_has_pending_irq()
> > 
>  >...
>  >context_switch(): nothing
> > 
> > The other is to do the "blocking" stuff in the context switch
> > (similar
> > to what's done now):
> > 
> > vcpu_block()
>   >set(_VPF_blocked)
>   >local_events_need_delivery()
>     >hvm_vcpu_has_pending_irq()
>   >...
> > context_switch
>    >v->arch.block()
>     >- Add v to pcpu.pi_blocked_vcpu
>     >- NV => pi_wakeup_vector
> >
> > [...]
> >
> > > > thing I like about the first one is that it makes PI blocking
> > > > the
> > same as normal blocking -- everything happens in the same place; so
> > the
> > code is cleaner and easier to understand.
> > 
> > The thing I like about the second one is that it cleverly avoids
> > having
> > to do all the work of adding the vcpu to the list and then
> > searching to
> > remove it if the vcpu in question gets woken up on the way to being
> > blocked.  So the code may end up being faster for workloads where
> > that
> > happens frequently.
> 
>  But wouldn't such an optimization argument apply to some/all other
> blocking ways too? I.e. shouldn't, if we were to go that route, other
> early wakeups get treated equally? 
>
Good point, actually.

However, with "regular blockings" this would probably be less of an
optimization.

In fact, in PI case, George's solution 2 is, potentially, avoiding
having to switch the descriptor and manipulating the list of blocked
vcpus.
In regular blockings there are no such things. We may be able to avoid
a context switch, but that also depends, AFAICT, on when the interrupt
exactly happens and/or is notified (i.e., before the call to schedule()
as opposed to after that and before the actual __context_switch()). I'm
not saying this wouldn't be better, but it's certainly optimizing less
than in the PI case.

> Unless that's wrong thinking of mine
> or being implemented that way, I'd clearly favor option 1 for
> consistency
> reasons.
> 
As said, me too. Perhaps we can go for option 1, which is simpler,
cleaner and more consistent, considering the current status of the
code. We can always investigate, in future, whether and how to
implement the optimization for all the blockings, if beneficial and fea
sible, or have them diverge, if deemed worthwhile.

Regards,
Dario
--
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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