Re: questions about ack_intr_on_exit and preemption hooks

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Stephane Eranian wrote:
> Avi,
>
> A couple of months back, we had a discussion about PMU virtualization
> and the difficulty I encountered trying to catch the PMU interrupt
> vector in kvm on VM-exit. KVM does not set ack_intr_on_intr. Would
> you mind reminding me of the reason for this?
>
>   

If we ack the interrupt, we have to dispatch it ourselves, most likely
by faking an 'int $xx' instruction.

By not acking the interrupt, we allow it do be delivered through the
host IDT whether we are in guest mode or not.

> On the topic of scheduler hooks for use by KVM, I think this
> would help me also because it means I would not have to save
> and restore the PMU registers on all VM-exit/VM-entry. At least
> that is what I understood based on the discussion. Those registers
> only need to be saved/restore when the KVM process is context switched.
>   

If you want to monitor the guest, then I think you need to switch
immediately on exit/entry, using the hardware msr switching mechanism. 
Otherwise, host events will be counted against the guest counters.

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


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