I just use DOS program as an example, for any program, if it wants to go
into VM86 mode, it is very easy, just calls i386_vm86() to initailize its
VM86 pcb extension, setups some memory area, then call sigreturn() to turn
into VM86 mode.
  I think global in_vm86call flags is a bug under SMP, it creates a race
condition. suppose this scenario:
  CPU A is running a simple VM86 code program.
  CPU B is running vm86_intcall() by some kernel driver (vesa module ?)
  CPU B set in_vm86call = 1
  CPU A gets a fault trap.
  CPU A runs trap(), and find that in_vm86call is set and handles the trap
        as  it is running vm86_intcall(), but it is not true and make a mess.
David Xu

--- Jonathan Lemon <[EMAIL PROTECTED]> wrote:
> On Sat, Jul 06, 2002 at 05:15:26AM -0700, David Xu wrote:
> > 
> > I don't know how DOS emulating program works, but if it let DOS
> > program run in VM86 mode, the in_vm86call global flag can prevent
> > one CPU to run VM86 BIOS call and another CPU run DOS VM86 code, 
> > because it can not distinct which CPU the kernel is calling BIOS
> > and which CPU is running VM86 DOS code, under SMP this is a problem.
> > for exapmle, vesa module running on first CPU is calling VM86 BIOS,
> > and second CPU is running DOS program, the DOS program maybe simply
> > executes a privilege instruction to trigger trap, and the CPU
> > will see itself calling VM86 BIOS, but it shouldn't.
> The virtual vm86 mode that doscmd(1) uses is different than the
> vm86 bios calls (or bios16, or bios32) which allows direct execution
> of BIOS code.  They do not have much in common.  doscmd() does not
> directly execute any of the BIOS code; it provides its own BIOS
> emulator.
> The scenario you postulate above cannot exist.
> -- 
> Jonathan

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