Re: [Xen-devel] Re: [PATCH 1/4] CPU online/offline support in Xen

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 11/9/08 15:32, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>>> I note that the VCPU currently running on the offlined CPU continues to run
>>> there even after __cpu_disable(), and until that CPU does a final run
>>> through the scheduler soon after. I hope it does not matter there is one
>>> vcpu with v->processor == offlined_cpu for a short while
>> 
>> This is not acceptable regarding to machine check. When Dom0 offlines a
>> defect cpu, nothing may continue on it or silent data corruption occurs.
> 
> It doesn't run for unbounded time. The offline CPU will immediately run
> softirq work as the very next thing it does, causing a run through the
> scheduler, where it will 100% definitely pick up the idle vcpu and hence
> play dead in the idle loop.
> 
> This won't be guaranteed synchronous with a offline request via a hypercall
> though. By which I mean, that hypercall may return but the last vcpu may not
> stop running on the cpu until some tiny amount of time later.

Actually I'm wrong on this. __cpu_die() will ensure the CPU really is
totally offline before returning. So calls to cpu_down() really are
synchronous already.

 -- Keir



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