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

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

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 <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".
>>
> 
> 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)?

Eric

 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
> 
>