Re: [Qemu-devel] [PATCH] qemu: msi irq allocation api

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 21, 2009 at 03:38:56PM +0300, Avi Kivity wrote:
> Paul Brook wrote:
>>> Instead of writing directly, let's abstract it behind a qemu_set_irq().
>>> This is easier for device authors.  The default implementation of the
>>> irq callback could write to apic memory, while for kvm we can directly
>>> trigger the interrupt via the kvm APIs.
>>>     
>>
>> I'm still not convinced.
>>
>> A tight coupling between PCI devices and the APIC is just going to 
>> cause us problems later one. I'm going to come back to the fact that 
>> these are memory writes so once we get IOMMU support they will 
>> presumably be subject to remapping by that, just like any other memory 
>> access.
>>   
>
> I'm not suggesting the qemu_irq will extend all the way to the apic.   
> Think of it as connecting the device core with its interrupt unit.
>
>> Even ignoring that, qemu_irq isn't really the right interface. A MSI is a one-
>> off event, not a level state. OTOH stl_phys is exactly the right interface.
>>   
>
> The qemu_irq callback should do an stl_phys().  The device is happy  
> since it's using the same API it uses for non-MSI.  The APIC is happy  
> since it isn't connected directly to the device.  stl_phys() is happy  
> since it sees more traffic and can serve more ads.  kvm is happy since  
> it can hijack the callback to throw the interrupt directly into the 
> kernel.

I like the monetization angle here.

>> The KVM interface should be contained within the APIC implementation.
>>   
>
> Tricky, but doable.

I'd rather keep it simple.

-- 
MST