Re: Adding MSI support for virtio-pci to QEMU as Xen backend on ARM

[Alex Bennée <alex.bennee@xxxxxxxxxx>]

Stefano Stabellini <sstabellini@xxxxxxxxxx> writes:

> +Stewart
>
> On Fri, 8 Dec 2023, Mykyta Poturai wrote:
>> Hello everyone,
>> 
>> I am currently working on adding MSI support to virtio-pci on ARM with Xen.
>
> Excellent!
>
>
>> As far as I understand QEMU Xen ARM machine that is used for
>> virtio-pci device emulation does not initialize any interrupt
>> controllers. And that makes it somewhat unclear what is the best
>> approach to adding the MSI support. So, I would appreciate some
>> thoughts on this.
>> To trigger the MSI interrupt in Xen guests we basically need to call a 
>> single DMOP, which I am also implementing.
>
> You should be able to call xen_inject_msi
>
>
>> So far, I have come up with 3 possible approaches:
>> 1. Hooking up the existing GIC code to the Xen Arm machine and extending it 
>> with the DMOP call.
>
> No, I would rather avoid this for a couple of reasons. In reality, the
> GIC is emulated by Xen, not QEMU. So letting QEMU think that QEMU is
> emulating a GIC is more error prone than the alternatives and also it
> would allocate resources needlessly.
>
>
>> 2. Writing some new barebones GIC implementation with only the required 
>> methods (something similar to xen_apic.c).
>> 3. Just hooking up the DMOP call to the existing PCI code based on 
>> xen_enabled() check.
>
> I would rather not introduce any more if (xen_enabled()) checks beyond
> the ones we already have unless it is the only option. So I would go
> with option 2). Even better if we could reuse xen_apic.c. xen_apic.c is
> just a stub, which is basically the same thing that we need here.

I guess the APIC part of the name is part of the historical baggage of
x86 - really it's just a xenpv-irq controller?

Anyway yeah I think just move it from hw/i386/xen to hw/xen and maybe
give it a new name with an alias to the old one we can deprecate over
time?

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro