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Single stepping Windows 10 bootloader results in Assertion `ret < cpu->num_ases && ret >= 0' failed.
Description of problem:
When I am trying to debug Windows bootloader, I see an assertion error in QEMU when single stepping some instructions in SeaBIOS.
```
qemu-system-i386: ../hw/core/cpu-sysemu.c:77: cpu_asidx_from_attrs: Assertion `ret < cpu->num_ases && ret >= 0' failed.
```
Steps to reproduce:
1. Download / construct `w.img`, see above
2. Start QEMU using `./qemu-system-i386 --drive media=disk,file=w.img,format=raw,index=1 -s -S -enable-kvm`
3. Start GDB using `gdb --ex 'target remote :::1234' --ex 'hb *0x7c00' --ex c --ex 'si 1000' --ex q`
4. See error message
Additional information:
The GDB script first breaks at 0x7c00, then tries to execute 1000 instructions using single step (`si`). On my machine, after executing around 772 instructions, the assertion error in QEMU happens.
Here is an interactive GDB session on my machine.
```
(gdb) hb *0x7c00
Hardware assisted breakpoint 1 at 0x7c00
(gdb) c
Continuing.
Breakpoint 1, 0x00007c00 in ?? ()
(gdb) d
Delete all breakpoints? (y or n) y
(gdb) si 770
0x000f7d7b in ?? ()
(gdb) x/10i $eip
=> 0xf7d7b: mov $0x7d85,%ebx
0xf7d80: out %al,$0xb2
0xf7d82: pause
0xf7d84: hlt
0xf7d85: mov %bp,%sp
0xf7d88: jmp 0xf7dd1
0xf7d8a: mov %cx,%si
0xf7d8d: mov $0x1,%ax
0xf7d91: add %al,(%eax)
0xf7d93: callw 0x6b66
(gdb) si
0x000f7d80 in ?? ()
(gdb) info reg
eax 0xb5 181
ecx 0x5678 22136
edx 0x0 0
ebx 0x7d85 32133
esp 0xe96d4 0xe96d4
ebp 0xfed4 0xfed4
esi 0xe0346 918342
edi 0xefd91 982417
eip 0xf7d80 0xf7d80
eflags 0x6 [ IOPL=0 PF ]
cs 0x8 8
ss 0x10 16
ds 0x10 16
es 0x10 16
fs 0x10 16
gs 0x10 16
fs_base 0x0 0
gs_base 0x0 0
k_gs_base 0x0 0
cr0 0x11 [ ET PE ]
cr2 0x0 0
cr3 0x0 [ PDBR=0 PCID=0 ]
cr4 0x0 [ ]
cr8 0x0 0
efer 0x0 [ ]
...
mxcsr 0x1f80 [ IM DM ZM OM UM PM ]
(gdb) si
0x000f7d82 in ?? ()
(gdb) info reg
eax 0xb5 181
ecx 0x5678 22136
edx 0x0 0
ebx 0x7d85 32133
esp 0xe96d4 0xe96d4
ebp 0xfed4 0xfed4
esi 0xe0346 918342
edi 0xefd91 982417
eip 0xf7d82 0xf7d82
eflags 0x6 [ IOPL=0 PF ]
cs 0x8 8
ss 0x10 16
ds 0x10 16
es 0x10 16
fs 0x10 16
gs 0x10 16
fs_base 0x0 0
gs_base 0x0 0
k_gs_base 0x0 0
cr0 0x11 [ ET PE ]
cr2 0x0 0
cr3 0x0 [ PDBR=0 PCID=0 ]
cr4 0x0 [ ]
cr8 0x0 0
efer 0x0 [ ]
...
mxcsr 0x1f80 [ IM DM ZM OM UM PM ]
(gdb) si
Remote connection closed
(gdb)
```
This bug was first incorrectly filed in KVM's bug tracker at <https://bugzilla.kernel.org/show_bug.cgi?id=216003>.
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