Re: [Qemu-devel] [PATCH 17/23] hyperv: add synic message delivery

[Roman Kagan]

On Wed, Jun 14, 2017 at 05:32:12PM +0200, Paolo Bonzini wrote:
> 
> 
> On 14/06/2017 17:28, Roman Kagan wrote:
> > On Wed, Jun 14, 2017 at 05:08:02PM +0200, Paolo Bonzini wrote:
> >>
> >>
> >> On 06/06/2017 20:19, Roman Kagan wrote:
> >>> +    sint_route->msg_status = ret;
> >>> +    /* notify the msg originator of the progress made; if the slot was 
> >>> busy we
> >>> +     * set msg_pending flag in it so it will be the guest who will do 
> >>> EOM and
> >>> +     * trigger the notification from KVM via sint_ack_notifier */
> >>> +    if (ret != -EAGAIN) {
> >>> +        qemu_bh_schedule(sint_route->msg_bh);
> >>> +    }
> >>
> >> It may be faster to use aio_bh_schedule_oneshot, depending on the number
> >> of devices.
> > 
> > Messages aren't used on fast paths, they are only exchanged at device
> > setup/teardown.  So I cared more about readability than speed here.
> 
> Then you really want to use aio_bh_schedule_oneshot, because bottom
> halves incur a (small) cost even when you don't use them: each iteration
> of the event loop visits the list of bottom halves.

I didn't realize that (yes that's easy to see in the code but the API
didn't suggest I needed to ;).

> Persistent bottom halves, thus, are only a good idea if they are
> expected to trigger very often on a busy VM.  If this bottom half is
> only triggered at setup/teardown, it shouldn't use them.

Thanks for pointing that out; we'll have to adjust our vmbus code too as
we used a persistent bottom half for message interactions there too.

Roman.