linux-kernel - Re: [KVM PATCH v2 0/2] irqfd: use POLLHUP notification for close()

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Message-ID: <20090612040837.GA8862@redhat.com>
Date:	Fri, 12 Jun 2009 07:08:37 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, paulmck@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org
Subject: Re: [KVM PATCH v2 0/2] irqfd: use POLLHUP notification for close()

[ Resending with correct address for Davide. Pls don't reply
  to the original one, you'll get bounces. ]

On Thu, Jun 04, 2009 at 08:48:02AM -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.
> 
> [Review notes:
>       *) Paul has looked at the SRCU design and, to my knowledge, didn't find
>          any holes.
>       *) Michael, Avi, and myself agree that while the removal of the DEASSIGN
>          vector is not desirable, the fix on close() is more important in
>          the short-term.  We can always add DEASSIGN support again in the
> 	 future with a CAP bit.
> ]

So, I've been thinking about this, and this approach has another
problem: it depends on pollhup on close which is AFAIK an
eventfd-specific feature. This will prevent us from supporting polling
other useful file types, such as sockets and pipes, down the road, with
this interface.

And there's DEASSIGN issue which is needed for migration and MSI vector
remapping.

I didn't realise these implications when I suggested deassign on close.
To me, it now looks like we are better off reverting this patch.
We can later add 'deassign on close' support with CAP bit after all :)

Avi, Gregory, what's your take?

-- 
MST
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