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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

+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.