On Tue, Jan 28, 2020 at 02:53:25PM +0100, Auger Eric wrote: > 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.au...@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.
So it should be backward compatible to switch to probing. Just try gicv2 first. If it works, then use it. Thanks, drew
