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)?
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 > >
