Re: Remove APIC lock

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

Dong, Eddie wrote:
> Avi:
>
>     apic->lock is used in many place to avoid race condition with apic
> timer call back
>     function which may run on different pCPU. This patch migrate the
> apic timer to
>     same CPU with the one VP runs on, thus the lock is no longer
> necessary.
>
> thx,eddie
>
>   


What about preemption:

- vcpu executes lapic code in qemu process context
- process is preempted
- timer fires, touches lapic code

Furthermore, I question the necessity of this.  Taking a spinlock is a
couple dozen cycles on modern processors.  Entering the guest is a
couple thousand.  So what are we saving?

(migrating the timer is a good thing though).

A different approach might be to wake up the vcpu like we do for irr,
with kvm_vcpu_kick(), and let the timer code be handled in process
context, so no locks need be taken.

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


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