Re: [Qemu-devel] qemu-kvm upstreaming: Do we need -no-kvm-pit and -no-kvm-pit-reinjection semantics?

[Jan Kiszka]

On 2012-01-19 18:25, Marcelo Tosatti wrote:
> On Thu, Jan 19, 2012 at 09:33:51AM +0100, Jan Kiszka wrote:
>> Hi all,
>>
>> I've finished a first version of cleaned-up in-kernel KVM PIT support.
>> That will be rolled out once the base support for irqchip has been merged.
>>
>> I'm now wondering if and how to model two control knobs we have in qemu-kvm:
>>
>>  o -no-kvm-pit, ie. disable the in-kernel PIT even when {A,IOA,}PIC
>>    are kernel based (default: off, ie. use in-kernel PIT)
> 
> It can be useful for debugging.

When was it last? The kernel model should be stable by now, just
possibly incomplete. But then there is still the full emulation in user
mode, available via kernel_irqchip=off.

We can control it, the question is if we really need the granularity.

> 
>>  o -no-kvm-pit-reinjection, ie. control over the lost ticks reinjection
>>    logic in the kernel (default: off, ie. do reinject)
> 
> If the guest kernel does not compensate for lost ticks, reinjection is 
> needed. Otherwise, it might cause problems.

That's why it's /on/ by default? What problems does it cause, and in
which scenarios? Can't they be fixed?

Jan

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