Panic in efi_get_time() on EPCY system when booting

Dear all,

r337761 panics on boot with a GENERIC kernel on a EPYC system:

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FreeBSD 12.0-ALPHA1 #3 r337761: Tue Aug 14 17:59:05 CEST 2018
    tuexen_at_epyc.nplab.de:/usr/home/tuexen/head/sys/amd64/compile/TCP amd64
FreeBSD clang version 6.0.1 (tags/RELEASE_601/final 335540) (based on LLVM 6.0.1)
WARNING: WITNESS option enabled, expect reduced performance.
VT(efifb): resolution 1024x768
CPU: AMD EPYC 7551P 32-Core Processor                (2000.05-MHz K8-class CPU)
  Origin="AuthenticAMD"  Id=0x800f12  Family=0x17  Model=0x1  Stepping=2
  Features=0x178bfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,MMX,FXSR,SSE,SSE2,HTT>
  Features2=0x7ed8320b<SSE3,PCLMULQDQ,MON,SSSE3,FMA,CX16,SSE4.1,SSE4.2,MOVBE,POPCNT,AESNI,XSAVE,OSXSAVE,AVX,F16C,RDRAND>
  AMD Features=0x2e500800<SYSCALL,NX,MMX+,FFXSR,Page1GB,RDTSCP,LM>
  AMD Features2=0x35c233ff<LAHF,CMP,SVM,ExtAPIC,CR8,ABM,SSE4A,MAS,Prefetch,OSVW,SKINIT,WDT,TCE,Topology,PCXC,PNXC,DBE,PL2I,MWAITX>
  Structured Extended Features=0x209c01a9<FSGSBASE,BMI1,AVX2,SMEP,BMI2,RDSEED,ADX,SMAP,CLFLUSHOPT,SHA>
  XSAVE Features=0xf<XSAVEOPT,XSAVEC,XINUSE,XSAVES>
  AMD Extended Feature Extensions ID EBX=0x1007<CLZERO,IRPerf,XSaveErPtr>
  SVM: NP,NRIP,VClean,AFlush,DAssist,NAsids=32768
  TSC: P-state invariant, performance statistics
real memory  = 137438953472 (131072 MB)
avail memory = 133661786112 (127469 MB)
Event timer "LAPIC" quality 600
ACPI APIC Table: < >
FreeBSD/SMP: Multiprocessor System Detected: 64 CPUs
FreeBSD/SMP: 1 package(s) x 4 groups x 2 cache groups x 4 core(s) x 2 hardware threads
random: unblocking device.
ioapic0: Changing APIC ID to 128
ioapic1: Changing APIC ID to 129
ioapic2: Changing APIC ID to 130
ioapic3: Changing APIC ID to 131
ioapic4: Changing APIC ID to 132
ioapic0 <Version 2.1> irqs 0-23 on motherboard
ioapic1 <Version 2.1> irqs 24-55 on motherboard
ioapic2 <Version 2.1> irqs 56-87 on motherboard
ioapic3 <Version 2.1> irqs 88-119 on motherboard
ioapic4 <Version 2.1> irqs 120-151 on motherboard
Launching APs: 14 6 52 16 28 44 42 36 20 13 37 4 54 15 21 5 55 18 30 8 26 9 29 58 53 10 56 38 31 11 49 46 22 48 43 12 35 45 41 23 39 51 32 24 27 61 63 33 1 62 60 7 59 40 34 47 2 3 17 19 50 25 57
Timecounter "TSC" frequency 2000054700 Hz quality 1000
random: entropy device external interface
netmap: loaded module
[ath_hal] loaded
module_register_init: MOD_LOAD (vesa, 0xffffffff81126100, 0) error 19
kbd1 at kbdmux0
random: registering fast source Intel Secure Key RNG
random: fast provider: "Intel Secure Key RNG"
nexus0
panic: mutex pmap not owned at ../../../amd64/amd64/efirt_machdep.c:268
cpuid = 60
time = 1
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff826ce810
vpanic() at vpanic+0x1a3/frame 0xffffffff826ce870
panic() at panic+0x43/frame 0xffffffff826ce8d0
__mtx_assert() at __mtx_assert+0xb4/frame 0xffffffff826ce8e0
efi_arch_enter() at efi_arch_enter+0x30/frame 0xffffffff826ce910
efi_get_time() at efi_get_time+0xbd/frame 0xffffffff826ce960
efirtc_probe() at efirtc_probe+0x17/frame 0xffffffff826ce990
device_probe_child() at device_probe_child+0x164/frame 0xffffffff826ce9f0
device_probe() at device_probe+0x98/frame 0xffffffff826cea20
device_probe_and_attach() at device_probe_and_attach+0x32/frame 0xffffffff826cea50
bus_generic_attach() at bus_generic_attach+0x18/frame 0xffffffff826cea70
device_attach() at device_attach+0x3f3/frame 0xffffffff826ceab0
device_probe_and_attach() at device_probe_and_attach+0x71/frame 0xffffffff826ceae0
bus_generic_new_pass() at bus_generic_new_pass+0xdd/frame 0xffffffff826ceb10
bus_set_pass() at bus_set_pass+0x8c/frame 0xffffffff826ceb40
configure() at configure+0x9/frame 0xffffffff826ceb50
mi_startup() at mi_startup+0x118/frame 0xffffffff826ceb70
btext() at btext+0x2c
KDB: enter: panic
[ thread pid 0 tid 100000 ]
Stopped at      kdb_enter+0x3b: movq    $0,kdb_why
db> 

Any idea what is wrong?

Best regards
Michael