[Bug 1902267] [NEW] CPU not support 32-bit stack in 32-bit unreal mode

[CandyMan]

Public bug reported:

QEMU version 5.0.0 supports 32-bit and 16-bit unreal mode. Great!
Unfortunately, QEMU does not support 32-bit stack in unreal 32-bit mode.
After the INT instruction, the stack is switched to 16-bit, which should not be 
the case. 
At BOCHS, my code works 100%. At QEMU not works.

Sample code to find out:

use32
cli
mov ax,cs
shl eax,16
mov ax,NewInt80h
mov [IDT32+4*80h],eax
mov edx,esp
mov esp,0x10000
int 80h
NewInt80h:
xchg esp,edx
cmp edx,0x10000-6
jnz IsStack16Bit

Stack selector loaded from GDT:
GDT:
real32_GDT            
dq 0
dw 0xFFFF,0x0000,9A00h,0xCF     ; 32-bit code descriptor
dw 0xFFFF,0x0000,9200h,0x8F     ;   4 GB data descriptor
dw 0xFFFF,0x0000,9A00h,0x00     ; 16-bit code descriptor
dw 0xFFFF,0x0000,9200h,0xCF     ; 32-bit data descriptor stack

** Affects: qemu
     Importance: Undecided
         Status: New

** Attachment added: "my operating system"
   
https://bugs.launchpad.net/bugs/1902267/+attachment/5429520/+files/FLATDOS.IMG

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https://bugs.launchpad.net/bugs/1902267

Title:
  CPU not support 32-bit stack in 32-bit unreal mode

Status in QEMU:
  New

Bug description:
  QEMU version 5.0.0 supports 32-bit and 16-bit unreal mode. Great!
  Unfortunately, QEMU does not support 32-bit stack in unreal 32-bit mode.
  After the INT instruction, the stack is switched to 16-bit, which should not 
be the case. 
  At BOCHS, my code works 100%. At QEMU not works.

  Sample code to find out:

  use32
  cli
  mov ax,cs
  shl eax,16
  mov ax,NewInt80h
  mov [IDT32+4*80h],eax
  mov edx,esp
  mov esp,0x10000
  int 80h
  NewInt80h:
  xchg esp,edx
  cmp edx,0x10000-6
  jnz IsStack16Bit

  Stack selector loaded from GDT:
  GDT:
  real32_GDT            
  dq 0
  dw 0xFFFF,0x0000,9A00h,0xCF     ; 32-bit code descriptor
  dw 0xFFFF,0x0000,9200h,0x8F     ;   4 GB data descriptor
  dw 0xFFFF,0x0000,9A00h,0x00     ; 16-bit code descriptor
  dw 0xFFFF,0x0000,9200h,0xCF     ; 32-bit data descriptor stack

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