Re: [KVM PATCH v3 2/2] kvm: add support for irqfd via eventfd-notification interface

[Avi Kivity <avi@redhat.com>]

Michael S. Tsirkin wrote:
> On Sun, May 03, 2009 at 07:59:40PM +0300, Avi Kivity wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> On Mon, Apr 27, 2009 at 02:33:34PM -0400, Gregory Haskins wrote:
>>>   
>>>       
>>>> This allows an eventfd to be registered as an irq source with a guest.  Any
>>>> signaling operation on the eventfd (via userspace or kernel) will inject
>>>> the registered GSI at the next available window.
>>>>
>>>> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
>>>>     
>>>>         
>>> If we ever want to use this with e.g. MSI-X emulation in guest, and want
>>> to be stricly compliant to MSI-X, we'll need a way for guest to mask
>>> interrupts, and for host to report that a masked interrupt is pending.
>>> Ideally, all this will be doable with a couple of mmapped pages to avoid
>>> vmexits/system calls.
>>>
>>>   
>>>       
>> We could do this in two ways:
>>
>> - move msix entry emulation into the kernel
>>     
>
> It's not too bad IMO: MSIX is just a table with a list
> of vectors, you check the mask bit on each interrupt,
> if masked mark vector pending and poll until unmasked.
>   

Right, but it's more and more core, and more and more bugs.  Bugs in the 
kernel are more expensive to fix and get to users.

>   
>> - require the device to support replacing its irqfd, and juggle it like so:
>>   - guest disables msi
>>       - replace device model fd with eventfd belonging to us
>>       - when the device fires its eventfd, set the irq pending bit
>>   - guest enables msi
>>        - if the pending bit is set, fire the interrupt?
>>        - replace device model fd with the real irqfd
>>     
>
> Looks like a lot of code. No?
>   

We'll need exactly the same code if we do it in the kernel.  The only 
addition is replacing the fd.

>> I'm leaning towards the latter, though it's not an easy call.
>>     
>
> Actually there's a third option: add KVM_MASK_IRQ, KVM_UNMASK_IRQ ioctls
> which will block/unblock guest from getting interrupt on this irq,
> whatever the source.  Interrupts are queued in kernel while masked. A
> third ioctl KVM_PENDING_IRQS will return the status for a set if IRQs.
> qemu would call these ioctls when guest edits the MSIX vector control or
> reads the pending bit array.
>   

I think this is the best option.

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