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

From: Pavel Fedin
Subject: Re: [Qemu-devel] [PATCH v2] Add virt-v3 machine that uses GIC-500
Date: Tue, 12 May 2015 14:15:46 +0300


> 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 

> (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

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