Re: [question] hw/arm/virt: about the default gic-version in accelerated mode

[Andrew Jones]

On Tue, Jan 28, 2020 at 10:52:50AM +0000, Peter Maydell wrote:
> On Tue, 28 Jan 2020 at 10:47, Auger Eric <address@hidden> wrote:
> > When arm virt machine is run in accelerated mode with "-cpu host
> > -machine virt", the default gic version is 2.
> >
> > I understand the rationale with TCG where we don't have MSI ITS
> > emulation along with GICv3 so we need to choose GICv2 to get GICv2M
> > functionality.
> >
> > However in KVM mode, I would have expected to see the host GIC probed to
> > set the same version on guest. Indeed most of our HW now have GICv3
> > without GICv2 compat mode so our default values lead to weird traces:
> >
> > "
> > qemu-system-aarch64: PMU: KVM_SET_DEVICE_ATTR: Invalid argument
> > qemu-system-aarch64: failed to set irq for PMU
> > "
> >
> > I would like to propose a patch to improve those errors and also suggest
> > a hint. But I also wanted to know whether you would accept to change the
> > default value with KVM and choose the host version instead of 2. For TCG
> > we would keep v2.
> 
> As with the -cpu option, the default is there for command
> line backward compatibility primarily. Even if we had
> better support for MSI ITS emulation we'd still leave
> the default at GICv2.
> 
> If you want "do the best you can, regardless of accelerator"
> that is "-cpu max -machine gic-version=max".
>

There is a case where we can probe without breaking backward
compatibility. That case is kvm-enabled and no gic-version
specified. The reason it would be safe to probe the GIC version
is because unless the host was a gicv2 host, then that command
line wouldn't have worked anyway. And, if it is a gicv2 host,
then the probing will come to the same conclusion as the
default and nothing will have changed.

Thanks,
drew