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Preserve the whole of %esp across prot_call(). We have to split this 

between the low half stored in the static variable rm_sp, and the high
half stored on the prot_call() stack, because:

  Just using the stack would screw up when a prot_call()ed routine
  executes a real_call(); it would have no way to find the current top of
  the RM stack.

  Extending rm_sp to rm_esp would not be safe, because the guarantee that
  rm_sp must return to the correct value by the time an external
  real-mode call returns applies only to %sp, not to %esp.
This commit is contained in:
Michael Brown 2006-05-06 19:44:23 +00:00
parent f8e087767b
commit fdb983d473
1 changed files with 14 additions and 7 deletions
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21
src/arch/i386/transitions/librm.S
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@ -229,10 +229,11 @@ real_to_prot:
*
* Switch from 32-bit protected mode with virtual addresses to 16-bit
* real mode. The protected-mode %esp is stored in pm_esp and the
* real-mode %ss:sp is restored from the saved rm_ss and rm_sp. All
* real-mode data segment registers are loaded from the saved rm_ds.
* Interrupts are *not* enabled, since we want to be able to use
* prot_to_real in an ISR. All other registers may be destroyed.
* real-mode %ss:sp is restored from the saved rm_ss and rm_sp. The
* high word of the real-mode %esp is set to zero. All real-mode data
* segment registers are loaded from the saved rm_ds. Interrupts are
* *not* enabled, since we want to be able to use prot_to_real in an
* ISR. All other registers may be destroyed.
*
* The return address for this function should be a 32-bit (sic)
* real-mode offset within .code16.
@ -284,7 +285,7 @@ p2r_jump_target:
/* Set up real-mode stack */
movw %bp, %ss
movw %dx, %sp
movl %edx, %esp
/* Set up real-mode data segments */
movw %cs:rm_ds, %ax
@ -321,7 +322,7 @@ rm_ds: .word 0
*
* All registers will be preserved across prot_call(), unless the C
* function explicitly overwrites values in ix86. Interrupt status
* will also be preserved. Gate A20 will be enabled.
* and GDT will also be preserved. Gate A20 will be enabled.
*
* Parameters:
* function : virtual address of protected-mode function to call
@ -384,7 +385,9 @@ prot_call:
.section ".text16"
.code16
1:
/* Reload GDT, restore registers and flags and return */
/* Reload GDT, restore registers and flags and return. Note
* that %esp is restored manually, since popal discards it.
*/
movw %sp, %bp
lgdt (%bp)
addw $12, %sp /* also skip %cs and %ss */
@ -393,6 +396,10 @@ prot_call:
popw %fs
popw %gs
popal
movl -20(%esp), %esp /* -20(%sp) is not a valid 80386 expression.
* -20(%esp) is safe because prot_to_real
* zeroes the high word of %esp, and interrupts
* are still disabled at this point. */
popfl
data32 ret