Re: [PATCH 5/5] ioeventfd: Introduce KVM_IOEVENTFD_FLAG_SOCKET

Avi Kivity
2011-07-14T12:48:58Z

On 07/14/2011 03:37 PM, Pekka Enberg wrote:
> Hi Avi,
>
> On Thu, Jul 14, 2011 at 2:54 PM, Avi Kivity<avi@redhat.com> wrote:
> >> I don't think it needs to be faster for *significant number* of users
> >> but yes, I completely agree that we need to make sure KVM gains more
> >> than the costs are.
> >
> > Significant, for me, means it's measured in a percentage, not as digits on
> > various limbs. 2% is a significant amount of users. 5 is not.
>
> Just to make my position clear: I think it's enough that there's one
> user that benefits as long as complexity isn't too high and I think
> x86/Voyager is a pretty good example of that. So while you can argue
> *for* complexity if there are enough users, when there's only few
> users, it's really about whether a feature adds significant complexity
> or not.

Everything adds complexity. And I think x86/voyager was a mistake.

> >> We want to use 8250 emulation instead of virtio-serial because it's
> >> more compatible with kernel debugging mechanisms. Also, it makes
> >> debugging virtio code much easier when we don't need to use virtio to
> >> deliver console output while debugging it. We want to make it fast so
> >> that we don't need to switch over to another console type after early
> >> boot.
> >>
> >> What's unreasonable about that?
> >
> > Does virtio debugging really need super-fast serial? Does it need serial at
> > all?
>
> Text mode guests should be super-fast, not virtio debugging. We want
> to use 8250 instead of virtio serial or virtio console (which we
> support, btw) because it's more compatible with Linux. Debugging
> virtio with 8250 has turned out to be useful in the past.

Use virtio-console when you're in production (it will be much much
faster than socket-mmio 8250), and 8250 when debugging.

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