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. Thanks, drew
