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Date: Mon, 29 Mar 2010 23:59:24 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH v3 1/1] Shared memory uio_pci driver
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To: Cam Macdonell <cam@cs.ualberta.ca>
Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>

On 03/28/2010 10:48 PM, Cam Macdonell wrote:
> On Sat, Mar 27, 2010 at 11:48 AM, Avi Kivity<avi@redhat.com>  wrote:
>    
>> On 03/26/2010 07:14 PM, Cam Macdonell wrote:
>>      
>>>        
>>>> I'm not familiar with the uio internals, but for the interface, an
>>>> ioctl()
>>>> on the fd to assign an eventfd to an MSI vector.  Similar to ioeventfd,
>>>> but
>>>> instead of mapping a doorbell to an eventfd, it maps a real MSI to an
>>>> eventfd.
>>>>
>>>>          
>>> uio will never support ioctls.
>>>        
>> Why not?
>>      
> Perhaps I spoke too strongly, but it was rejected before
>
> http://thread.gmane.org/gmane.linux.kernel/756481
>
> With a compelling case perhaps it could be added.
>    

Ah, the usual "ioctls are ugly, go away".

It could be done via sysfs:

   $ cat /sys/.../msix/max-interrupts
   256
   $ echo 4 > /sys/.../msix/allocate
   $ # subdirectories 0 1 2 3 magically appear
   $ # bind fd 13 to msix
   $ echo 13 > /sys/.../msix/2/bind-fd
   $ # from now on, msix interrupt 2 will call eventfd_signal() on fd 13

Call me old fashioned, but I prefer ioctls.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.