Re: [Alacrityvm-devel] [KVM PATCH v2 1/2] KVM: export lockless GSI attribute

Gregory Haskins
2009-10-28T13:20:00Z

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Avi Kivity wrote:
> On 10/26/2009 05:38 PM, Gregory Haskins wrote:
>>>> Instead of a lockless attribute, how about a ->set_atomic() method.
>>>> For
>>>> msi this can be the same as ->set(), for non-msi it can be a function
>>>> that schedules the work (which will eventually call ->set()).
>>>>
>>>> The benefit is that we make a decision only once, when preparing the
>>>> routing entry, and install that decision in the routing entry
>>>> instead of
>>>> making it again and again later.
>>>>
>>> Yeah, I like this idea. I think we can also get rid of the custom
>>> workqueue if we do this as well, TBD.
>>>
>> So I looked into this. It isn't straight forward because you need to
>> retain some kind of state across the deferment on a per-request basis
>> (not per-GSI). Today, this state is neatly tracked into the irqfd
>> object itself (e.g. it knows to toggle the GSI).
>>
>
> Yes, and it also contains the work_struct.
>
> What if we make the work_struct (and any additional state) part of the
> set_atomic() argument list? Does it simplify things?

Hmmm, that might not, but we could do a kmalloc(GFP_ATOMIC) for such
parameters. Considering this is just a safety net, perhaps this would
work fine.

>
>> So while generalizing this perhaps makes sense at some point, especially
>> if irqfd-like interfaces get added, it probably doesn't make a ton of
>> sense to expend energy on it ATM. It is basically a generalization of
>> the irqfd deferrment code. Lets just wait until we have a user beyond
>> irqfd for now. Sound acceptable?
>>
>
> I'll look at v3, but would really like to disentangle this.

Ok, I will see what I can do. I need at least a v4 to get rid of the
dependency on the now defunct v3:1/3 patch per yesterdays discussion.

Kind Regards,
-Greg

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