6.1/amd64 panic in cpu_switchto()

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

On an amd64 system running a 6.1 from November 2013 (yes, I need to update)
I got the following panic (manual OCR from screen photographs):

fatal protection fault in supervisor mode
trap type 4 code 0 rip ffffffff80100504 cs 8 rflags 10246 cr2  7f7ffffffe50 cpl8 rsp fffffe8055879670
kernel: protection fault trap, code=0
Stopped in pid 26911.26911 (mount) at	netbsd:cpu_switchto+0xf4:	wrmsr
db{4} bt
cpu_switchto() at netbsd:cpu_switchto+0xf4
kpreempt() at netbsd:kpreempt+0xe2
Xpreemtresume() at netbsd:Xpreemptresume+0x28
cpu_fsgs_zero() at netbsd:cpu_fsgs_zero+0x6a
setregs() at netbsd:setregs+0x7f
execve_runprog() at netbsd: execve_runprog+0x708
execve1() at netbsd: execve1+0x33
linux32_syscall() at netbsd:linux32_syscall+0xb9
db{4}> show registers
ds	1b0
es	c878
fs	1b0
gs	1
rdi	fffffe841978aa50
rsi	fffffe806cb3d780
rbp	fffffe80558796f0
rbx	fffffe811d84c2c0
rdx	fbcff3dd
rcx	c0000102
rax	fb50ffff
r8	0
r9	0
r10	0
r11	1
r12	fffffe806cb3d780
r13	fffffe811d84c2c0
r14	fffffe8055879d80
r15	0
rip	ffffffff80100504	cpu_switchto+0xf4
cs	8
rflags	10246
rsp	fffffe8055879670
ss	0
netbsd:cpu_switchto+0xf8	wrmsr
db{4}> reboot

After that, the machine was completely stuck, i.e., Ctrl-Alt-Esc didn't work.
I had to press reset.