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

Avi Kivity
2009-10-28T10:04:18Z

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?

> 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.

> In the meantime, I found a bug in the irq_routing code, so I will submit
> a v3 with this fix, as well as a few other things I improved in the v2
> series.
>
>

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