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

[Auger Eric <eric.auger@redhat.com>]

Hi Peter,

On 1/28/20 11:52 AM, Peter Maydell wrote:
> On Tue, 28 Jan 2020 at 10:47, Auger Eric <eric.auger@redhat.com> 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".

OK that's understood.

So I will just try to improve the above traces.

Thanks

Eric
> 
> thanks
> -- PMM
>