Re: [PATCH 3/8] KVM: Adds ability to preempt an executing VCPU

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

Gregory Haskins wrote:
>   
>> Later we'll have vcpu and thread_info point to each other and then you
>> can do that kind of optimization.
>>     
>
> I am not familiar with thread_info, but if it solves the dangling pointer problem, that sounds great.

It's fairly tricky, but it's the right way forward.  It means a 1:1 
association of a vcpu and a thread for the lifetime of the vcpu.

>   It sounds like you are in favor of leaving this optimization for a later time.  As long as you are ok with every interrupt related ioctl such as KVM_APIC_MSG, and KVM_ISA_INTERRUPT posting a signal to itself, we can pull this for now.

Perhaps you can disable this by noticing that you're injecting an 
interrupt now (another icky variable in struct kvm_vcpu).

>   Conversely, if the thread_info approach isn't hard, I would prefer to get this right now, as the double interrupt thing seems nasty to me.
>
> Alternatively, perhaps I can just replace irq.task with irq.pid?  And I could also replace irq.guest_mode with irq.guest_cpu.  I will then record the pid where today I record the task.  Likewise, I can extract the guest_cpu (using task_cpu(current)) where today I assign irq.guest_mode = 1.  That would effectively remove the dangling pointer problem while retaining the features that I like.  
>   

pid can dangle just the same as a task pointer, only much worse.

> Thoughts?
>
>   
>> Oh, and nobody said that the task waiting on the event is the same as
>> the task running the vcpu.
>>     
>
> I'm a little confused by this statement.  I don't use irq.task for assigning a target for the event.  That is all self contained in the eventfd.  Its true that some of the older designs used this as the send_sig() target, but the assumption there was we were posting a signal to the entire PID, not a specific TID.  That could have been a bad assumption, but its moot now.  Let me know if you meant something else.
>   

Strike that.  I was confused.

-- 
error compiling committee.c: too many arguments to function


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