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

[Andrew Jones]

On Tue, Jan 28, 2020 at 01:34:06PM +0100, Auger Eric wrote:
> Hi Drew,
> 
> On 1/28/20 1:29 PM, Andrew Jones wrote:
> > 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.
> Except if the host GICv3 has a GICv2 compat (which is pretty unlikely)?

Is there a way to probe that? If so, and the setting up of gicv2 on
a gicv3 host with the gicv2-compat is the same as setting up gicv2,
then we can just choose gicv2 to keep the command line compatibility.

Thanks,
drew