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Linux runs a basic sanity test to determine this. See: >>> https://github.com/torvalds/linux/blob/master/arch/x86/events/core.c#L268 >> >> So.. what do we do with all this?  :) >> >> I kinda see the intention of the original change, but it results in >> non-bootable systems (linux too, as I think you demonstrate above). >> > > Because of the sanity test, Linux will boot but not register the "cpu" PMU. > Perhaps others should also run a similar sanity test on PMU hardware rather > than making assumptions and failing to boot. After looking at kvm_msr and kvm_cpuid traces, I see that Windows is indeed getting #GPs upon accessing MSRs 0xc0010200 and 0xc0010201 but it is not at fault here since it expects these MSRs to be available as guest CPUID 0x80000001[ECX].PerfCtrExtCore is set. This feature bit should have been cleared by QEMU when "pmu=off". It boots up when "pmu=off" is combined with "perfctr-core=off" and "perfmon-v2=off". The missing pieces here are the following: https://lore.kernel.org/qemu-devel/20251111061532.36702-2-dongli.zhang@oracle.com/ https://lore.kernel.org/qemu-devel/20251111061532.36702-3-dongli.zhang@oracle.com/ Dongli, Zhao, I see a discussion about compat array at https://lore.kernel.org/qemu-devel/aUEXkDDOba+oZ4v+@intel.com/ Is this ready for the two patches above to be pulled in?