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

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Date:	Wed, 28 Oct 2009 09:30:46 -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 10/28/2009 03:19 PM, Gregory Haskins wrote:
>>> 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.
>>    
> 
> Can't you simply pass the same work_struct from irqfd as we use now?

Well, yes, of course, but I am not sure that buys us much in terms of
generalizing the code.  Unless I am misunderstanding, that would still
leave the impetus of the init/sync/cleanup to the irqfd code, at which
point we might as well just leave it entirely in irqfd anyway.  Or am I
misunderstanding you?

> 
>>>> 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.
>>    
> 
> There's another alternative - make ioapic and pic irq-safe by switching
> irq locking to spinlocks and using spin_lock_irqsave().
> 
> I've long opposed this since the ioapic loops on all vcpus when
> injecting some irqs and this will increase irqoff times with large
> guests.  But we don't have large guests now, and we need irq-safe
> injection in three places now:
> 
> - irqfd
> - pit - we now signal vcpu0 to handle the injection, but this has its
> problems
> - device assignment
> 
> so it may be better to have irq-safe injection, and deal with the loop
> later (would be good to have an idea how exactly).

Ok, perhaps I should just hold off on this series for now, then.  I can
submit the original "assume atomic safe" once the path is fully lockless.

-Greg

> 



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