Re: [RFC] Paravirt timer for KVM

Avi Kivity
2007-10-16T07:17:45Z

Glauber de Oliveira Costa wrote:
>>>>
>>>> Obviously people have figured out how to do dynticks on real x86
>>>> hardware, so I don't accept this reason. :)
>>>>
>>>>
>>> Using more advanced timers like the HPET.
>>>
>>>
>> I'd think there is no reason for virtual timer to be PIT like, which
>> is mostly due to historic reason.
>>
>> If we need to make it close to native timer device, HPET is much
>> better than PIT for virtual time to look like.
>>
>>
>
> Besides of course, the fact that you'd still have to keep the PIT
> timer around, for this same historical reason: It lives in an
> emulation layer that can't make too much assumptions about what's
> running on it, and it may well use the PIT exclusively. And we end up
> with two code bases to maintain.
>
> Obviously, as everything else, it's a trade-off. If the only benefit
> of an HPET like timer is its programability, then the paravirt timer
> solves this problem with less effort. If there are others, it might
> well be worth it.
>
>

Using the HPET means older kernels (and other operating systems)
immediately benefit. It also means we don't have to design the
interface. While it's usually harder to do full emulation, and you get
worse performance than paravirt, it has very real benefits.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.