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

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

Hi,

On 1/28/20 1:41 PM, Andrew Jones wrote:
> 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 <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)?
> 
> 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.
I think that if the host GICv3 is GICv2 compatible then you can create
both a KVM_DEV_TYPE_ARM_VGIC_V2 device and a KVM_DEV_TYPE_ARM_VGIC_V3
device. Otherwise you can only create a KVM_DEV_TYPE_ARM_VGIC_V3 KVM device.

Thanks

Eric

> 
> Thanks,
> drew
> 
>