Re: [PATCH] [RESEND] KVM:VMX: Add support for Pause-Loop Exiting

[Avi Kivity <avi@redhat.com>]

On 09/25/2009 11:43 PM, Joerg Roedel wrote:
> On Wed, Sep 23, 2009 at 05:09:38PM +0300, Avi Kivity wrote:
>    
>> We haven't sorted out what is the correct thing to do here.  I think we
>> should go for a directed yield, but until we have it, you can use
>> hrtimers to sleep for 100 microseconds and hope the holding vcpu will
>> get scheduled.  Even if it doesn't, we're only wasting a few percent cpu
>> time instead of spinning.
>>      
> How do you plan to find out to which vcpu thread the current thread
> should yield?
>    

We can't find exactly which vcpu, but we can:

- rule out threads that are not vcpus for this guest
- rule out threads that are already running

A major problem with sleep() is that it effectively reduces the vm 
priority relative to guests that don't have spinlock contention.  By 
selecting a random nonrunnable vcpu belonging to this guest, we at least 
preserve the guest's timeslice.

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