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

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Date:	Mon, 26 Oct 2009 09:25:10 -0400
From:	Gregory Haskins <gregory.haskins@...il.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	alacrityvm-devel@...ts.sourceforge.net
Subject: Re: [Alacrityvm-devel] [KVM PATCH v2 1/2] KVM: export lockless GSI
 attribute

Avi Kivity wrote:
> On 10/23/2009 04:38 AM, Gregory Haskins wrote:
>> Certain GSI's support lockless injecton, but we have no way to detect
>> which ones at the GSI level.  Knowledge of this attribute will be
>> useful later in the series so that we can optimize irqfd injection
>> paths for cases where we know the code will not sleep.  Therefore,
>> we provide an API to query a specific GSI.
>>
>>    
> 
> 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.

> 
>> +int kvm_irq_check_lockless(struct kvm *kvm, u32 irq)
>>    
> 
> bool kvm_irq_check_lockless(...)

We lose the ability to detect failure (such as ENOENT) if we do this,
but its moot if we move to the ->set_atomic() model, since this
attribute is no longer necessary and this patch can be dropped.

Kind Regards,
-Greg


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