linux-kernel - Re: [KVM PATCH v7 2/3] kvm: add support for irqfd via eventfd-notification interface

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Date:	Thu, 14 May 2009 23:35:52 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	viro@...IV.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [KVM PATCH v7 2/3] kvm: add support for irqfd via eventfd-notification
 interface

Davide Libenzi wrote:
> On Thu, 14 May 2009, Gregory Haskins wrote:
>
>   
>> Avi Kivity wrote:
>>     
>>> Gregory Haskins wrote:
>>>       
>>>> KVM provides a complete virtual system environment for guests, including
>>>> support for injecting interrupts modeled after the real
>>>> exception/interrupt
>>>> facilities present on the native platform (such as the IDT on x86).
>>>> Virtual interrupts can come from a variety of sources (emulated devices,
>>>> pass-through devices, etc) but all must be injected to the guest via
>>>> the KVM infrastructure.  This patch adds a new mechanism to inject a
>>>> specific
>>>> interrupt to a guest using a decoupled eventfd mechnanism:  Any legal
>>>> signal
>>>> on the irqfd (using eventfd semantics from either userspace or
>>>> kernel) will
>>>> translate into an injected interrupt in the guest at the next available
>>>> interrupt window.
>>>>
>>>> +
>>>> +static void
>>>> +irqfd_inject(struct work_struct *work)
>>>> +{
>>>> +    struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
>>>> +    struct kvm *kvm = irqfd->kvm;
>>>> +
>>>>   
>>>>         
>>> I think you need to ->read() from the irqfd, otherwise the count will
>>> never clear.
>>>       
>> Yeah, and this is a disavantage to using eventfd vs a custom anon-fd
>> implementation.
>>
>> However, the count is really only there for deciding whether to sleep a
>> traditional eventfd recipient which doesn't really apply in this
>> application.  I suppose we could try to invoke the read method (or add a
>> new method to eventfd to allow it to be cleared independent of the
>> f_ops->read() (ala eventfd_signal() vs f_ops->write()).  I'm not
>> convinced we really need to worry about it, though.  IMO we can just let
>> the count accumulate.
>>
>> But if you insist this loose end should be addressed, perhaps Davide has
>> some thoughts on how to best do this?
>>     
>
> The counter is 64bit, so at 1M IRQ/s will take about 585K years to 
> saturate. But from a symmetry POV, it may be better to clear it. Maybe 
> with a kernel-side eventfd_read()?
>   
Hi Davide,

I think ultimately that would be the direction to go.  I will defer to
Avi, but I think we have reached consensus that while its perhaps sloppy
to leave the counter untouched, we can back-burner this issue for now
and just let it accumulate indefinately.  If it becomes an issue down
the road we can always fix it then.

Thanks,
-Greg



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