Guest (baseline + trigger): DragonFly dfbsd 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026
    root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64   (clean-install snapshot)
Guest (fix build): v6.5-DEVELOPMENT with-src snapshot, single-fix kernel #1 via
    make -j6 nativekernel KERNCONF=X86_64_GENERIC (gcc80, -Werror)
Host CPU: Intel(R) Xeon(R) CPU E3-1275 v5 @ 3.60GHz (Skylake), KVM
QEMU: -enable-kvm -smp 6 -m 4096, QEMU_EXTRA="-cpu host" for all runs here
Stock qemu64 CPU: perfbias compiled in (kldload -> "already loaded or in kernel"),
    no machdep.perfbias0.hint (CPUID leaf 6 = 0) -> gated, inert
-cpu host (Skylake passthrough): hw.model=Intel Xeon E3-1275 v5,
    "Thermal and PM Features=0x4<ARAT>" (leaf 6 EAX), dmesg perfbias count 0,
    sysctl machdep.perfbias0.hint -> unknown oid
    => KVM masks CPUID.06H.ECX bit 3 (SETBH) for this guest; it does NOT
       virtualize MSR 0x1b0 (proven: rdmsr_safe/wrmsr_safe -> error 14)
Trigger: pbtest.ko (bsd.kmod.mk, -Werror, gcc80) kldload as root on -cpu host guest
    rdmsr_safe(MSR_IA32_ENERGY_PERF_BIAS=0x1b0) -> error 14, val 0
    wrmsr_safe(0x1b0, 7)            -> error 14
    raw wrmsr(0x1b0, 7)             -> Fatal trap 9 GPF in kernel mode,
        Stopped at pbtest_modevent+0xad: wrmsr, guest dead in DDB
perfbias compiled into stock kernel: sys/config/X86_64_GENERIC:173 "device perfbias"
Sysctl write privilege: kern_sysctl.c:1446-1447 (SYSCAP_NOSYSCTL_WR unless CTLFLAG_ANYBODY)