Re: [RFC][PATCH] qemu-kvm: Introduce writeback scope for cpu_synchronize_state

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> On 11/17/2009 04:12 PM, Jan Kiszka wrote:
>>>> The alternative would be a complex get&lock/put&unlock + a queue for
>>>> async events during the lock + an option to ignore what was queued when
>>>> doing a true reset. Back to square #1: we would still need the proposed
>>>> high-level interface to communicate the difference between replay and
>>>> drop queue.
>>>>    
>>>>       
>>> There's no need for get+lock / put+unlock; a normal get/put with the
>>>     
>> You need to track when to queue and when to apply directly. Call it lock
>> or call it something else.
>>   
> 
> You always queue.  When starting vcpu_run() or reading state to
> userspace you flush the queue.

Now I finally got your idea.

> 
> The hardware equivalent is posting APIC messages, and the core executing
> them.
> 
>>> addition that get flushes the queue suffices.  To make sure queued
>>> events don't affect set you need to stop the entire VM before setting
>>> state, but you need to do that anyway for non-rmw writes.
>>>
>>>     
>> Well, sounds good, but it will be a non-trivial change in the interface
>> semantics. At bare minimum, we would need a new mp_state interface. If
>> we would count mp_state to our new event structure (hmm...), then we
>> could confine the semantical changes to that new IOCTL pair. But how to
>> deal with existing KVM kernels with their mp_state interface? It's a bit
>> like the vcpu state thing: we are already down a specific road, and it's
>> hard to turn around.
>>   
> 
> I think we're not on the same page here.  As I see it, no interface
> change is needed at all.
> 
> It's true that existing kernels don't handle this properly, which is why
> I said I'm willing to treat it as a bug (and thus the -stable treatment
> etc.).  I admit it's a stretch since this is not going to be trivial
> (though I think less complex that you believe).
> 
> Putting mp_state into the events structure is reasonable regardless of
> this issue (and doable since we haven't pushed it to 2.6.33 yet).  But I
> want to understand why you think it's needed.
> 

That wouldn't be required anymore with the "always queue" policy.

But what would you queue at all? Only mp_state, nmi_pending and
sipi_vector? Or also all the relevant PIC and LAPIC states that might be
changed asynchronously?

Jan


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