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

From: Auger Eric
Subject: Re: [question] hw/arm/virt: about the default gic-version in accelerated mode
Date: Tue, 28 Jan 2020 13:34:06 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

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)?


 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

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