linux-kernel - Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

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Message-ID: <4A255484.6060401@novell.com>
Date:	Tue, 02 Jun 2009 12:34:12 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

Michael S. Tsirkin wrote:
> On Tue, Jun 02, 2009 at 12:14:15PM -0400, Gregory Haskins wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> On Tue, Jun 02, 2009 at 11:15:28AM -0400, Gregory Haskins wrote:
>>>   
>>>       
>>>> (Applies to kvm.git/master:25deed73)
>>>>
>>>> Please see the header for 2/2 for a description.  This patch series has
>>>> been fully tested and appears to be working correctly.  I have it as an RFC
>>>> for now because it needs Davide's official submission/SOB for patch 1/2, and
>>>> it should get some eyeballs/acks on my SRCU usage before going in.
>>>>
>>>> I will submit the updated irqfd userspace which eschews the deassign() verb
>>>> since we can now just use the close(fd) method alone.  I will also address
>>>> the userspace review comments from Avi.
>>>>     
>>>>         
>>> We are not killing the deassign though, do we?
>>>   
>>>       
>> Yes, it is not needed any more now that we have proper
>> release-notification from eventfd.
>>
>>     
>>> It's good to have that option e.g. for when we pass
>>> the fd to another process.
>>>   
>>>       
>> Passing the fd to another app should up the underlying file reference
>> count.  If the producer app wants to "deassign" it simply calls
>> close(fd) (as opposed to today where it calls DEASSIGN+close), but the
>> reference count will allow the consuming app to leave the eventfd's file
>> open.  Or am I misunderstanding you?
>>
>> -Greg
>>
>>
>>     
>
> I think we want to keep supporting the deassign ioctl. This, even though
> close overlaps with it functionally somewhat.
>
> This allows qemu to pass eventfd to another process/device, and then
> block/unblock interrupts as seen by that process by
> assigning/deassigning irq to it. This is much easier and lightweight
> than asking another process to close the fd and passing another fd
> later.
>
>   
Perhaps, but if that is the case we should just ignore this series and
continue with the DEASSIGN+close methodology since it already provides
that separation.  Trying to do a hybrid is just messy.

But in any case, I think that approach is flawed.  DEASSIGN shouldn't be
used as a mask in my opinion, and we shouldn't be reassigning a
channel's meaning under the covers like that.  If this is in fact a
valid use case, we should have a separate "GSI_MASK" type operation that
is independent of irqfd.  Likewise, we really should pass a new fd if
the gsi-routing is changing.  Today there is a tight coupling of
fd-to-gsi, and I think that makes sense to continue this association.

-Greg


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