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

Pekka Enberg
2011-07-14T12:37:19Z

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.

>> 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.

>> I thought you were convinced that KVM_CAP_MAX_VCPUS was reasonable. I
>> guess I misunderstood your position then.
>
> It's "okay", but no more.  If I were Linus I'd say it's scalability
> masturbation.

That's definitely not the intent and I suppose we have different
opinions on what's 'reasonably fast'.

Pekka