Re: [Qemu-devel] [PATCH] RFC kvm irqfd: add directly mapped MSI IRQ support

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 06/21/2013 04:12 PM, Benjamin Herrenschmidt wrote:
> On Fri, 2013-06-21 at 00:03 -0600, Alex Williamson wrote:
>> On Fri, 2013-06-21 at 15:12 +1000, Benjamin Herrenschmidt wrote:
>>> On Thu, 2013-06-20 at 22:46 -0600, Alex Williamson wrote:
>>>> Maybe you could add a device parameter to kvm_irqchip_add_msi_route so
>>>> that it can be implemented on POWER without this pci_bus_map_msi
>>>> interface that seems very unique to POWER.  Thanks,
>>>
>>> You mean unique to all non-x86 ? :-)
>>>
>>> I believe almost everybody eventually turn MSIs into "normal"
>>> interrupts...
>>>
>>> Most often than not, the logic to do so is in the PCI Host Bridge.
>>>
>>> The whole concept of passing the message address/data accross the
>>> user/kernel interface is an x86 crackpotery but as is the entire
>>> remapping/routing layer so ... :-)
>>
>> Regardless, this is exactly what kvm_irqchip_add_msi_route does.  It
>> says, here's an MSIMessage, give me an IRQ that sends that. 
> 
> Yes, and in our case, what happens is that the guest said to use "I want
> an MSI", we picked up an IRQ, and made up a message for it :-) The
> actual message address/data we use is a complete invention that only
> exists within qemu. So here we need to basically turn it back into an
> IRQ, which we might be able to do by ... just making the message (or
> part of the address) be the IRQ number or something like that.
> 
>>  In the x86
>> case, that means pick a free IRQ and program it to send that MSIMessage
>> when we hit the irqfd.  In the case of POWER it means lookup which IRQ
>> gets fired by that MSIMessage and return it.  In a non-accelerated QEMU
>> case I'd think msi_notify() would write the MSIMessage to this IRQ
>> remapper device and let it toggle the next qemu_irq down the line.  If
>> we ever add an IOMMU based IRQ remapper to the x86 model, we'd need
>> something similar.  Thanks,


Yes, I am changing IRQ to MSI message conversion (and some other things as
replacing per PHB MSI windows to one for a whole system) and I am going to
post something tonight.



-- 
Alexey