Re: [Qemu-devel] [PATCH v2] Add virt-v3 machine that uses GIC-500

[Pavel Fedin <p.fedin@samsung.com>]

 Hello!

> We are not. Support for GICv2 vs v3 should be dealt with
> by suitable machine properties

 I don't remember whether i clearly wrote about it... First i added a property, like -machine virt,gicv3=on. But then i decided to stick back to different machine name because libvirt does not have mechanism for passing machine options.

> (and by figuring out how
> we handle probing for which of the two the host kernel
> can provide us).

 This is tricky. I thought about it.
 Kernel offers us only KVM_CAP_IRQCHIP Boolean. But it does not tell us which irqchip. Possible variants are:
a) Host with GICv2 - this can only be v2.
b) Host with GICv3 with backwards compatibility option - this can be both
b) Host with GICv3 without compatibility option - this can only be v3.
 Perhaps we could do a probe in  kvm_arch_irqchip_create(), however i'm not sure what happens in the kernel when this is called. This call actually instantiates the device, but drops its fd. I suggest, the device is still created, and next attempt to do it will just return the same fd. And i don't know what happens if we create both GICs (i cannot test because i don't have machine capable of doing it).
 OTOH, if we don't have in-kernel acceleration, perhaps the good idea is stick to what the user wants, and use emulation. Yes, i know about problems with generic timer in this case, but this if different issue which can also be fixed.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia