[prefix] Use flat real mode for access to high memory

Use flat real mode rather than 16-bit protected mode for access to
high memory during installation.  This simplifies the code by reducing
the number of CPU modes we need to think about, and also increases the
amount of code in common between the normal and (somewhat
hypothetical) KEEP_IT_REAL methods of operation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

diff --git a/src/arch/i386/prefix/libprefix.S b/src/arch/i386/prefix/libprefix.S
index ecef56d..4decb01 100644 (file)
--- a/src/arch/i386/prefix/libprefix.S
+++ b/src/arch/i386/prefix/libprefix.S
@@ -233,156 +233,128 @@ print_kill_line:
        .size   print_kill_line, . - print_kill_line
 
 /****************************************************************************
- * pm_call (real-mode near call)
+ * flatten_real_mode
  *
- * Call routine in 16-bit protected mode for access to extended memory
+ * Set up 4GB segment limits
  *
  * Parameters:
- *   %ax : address of routine to call in 16-bit protected mode
+ *   none
  * Returns:
  *   none
  * Corrupts:
- *   %ax
- *
- * The specified routine is called in 16-bit protected mode, with:
- *
- *   %cs : 16-bit code segment with base matching real-mode %cs
- *   %ss : 16-bit data segment with base matching real-mode %ss
- *   %ds,%es,%fs,%gs : 32-bit data segment with zero base and 4GB limit
- *
+ *   none
  ****************************************************************************
  */
-
 #ifndef KEEP_IT_REAL
 
        /* GDT for protected-mode calls */
        .section ".prefix.lib", "awx", @progbits
        .align 16
-pm_call_vars:
-gdt:
-gdt_limit:             .word gdt_length - 1
-gdt_base:              .long 0
+flatten_vars:
+flatten_gdt:
+flatten_gdt_limit:     .word flatten_gdt_length - 1
+flatten_gdt_base:      .long 0
                        .word 0 /* padding */
-pm_cs:         /* 16-bit protected-mode flat code segment */
-       .equ    PM_CS, pm_cs - gdt
+flatten_cs:    /* 16-bit protected-mode flat code segment */
+       .equ    FLAT_CS, flatten_cs - flatten_gdt
        .word   0xffff, 0
        .byte   0, 0x9b, 0x8f, 0
-pm_ss:         /* 16-bit protected-mode flat stack segment */
-       .equ    PM_SS, pm_ss - gdt
+flatten_ss:    /* 16-bit protected-mode flat stack segment */
+       .equ    FLAT_SS, flatten_ss - flatten_gdt
        .word   0xffff, 0
        .byte   0, 0x93, 0x8f, 0
-pm_ds:         /* 32-bit protected-mode flat data segment */
-       .equ    PM_DS, pm_ds - gdt
-       .word   0xffff, 0
-       .byte   0, 0x93, 0xcf, 0
-gdt_end:
-       .equ    gdt_length, . - gdt
-       .size   gdt, . - gdt
+flatten_gdt_end:
+       .equ    flatten_gdt_length, . - flatten_gdt
+       .size   flatten_gdt, . - flatten_gdt
 
        .section ".prefix.lib", "awx", @progbits
        .align 16
-pm_saved_gdt:  
+flatten_saved_gdt:
        .long   0, 0
-       .size   pm_saved_gdt, . - pm_saved_gdt
+       .size   flatten_saved_gdt, . - flatten_saved_gdt
 
-       .equ    pm_call_vars_size, . - pm_call_vars
-#define PM_CALL_VAR(x) ( -pm_call_vars_size + ( (x) - pm_call_vars ) )
+       .equ    flatten_vars_size, . - flatten_vars
+#define FLATTEN_VAR(x) ( -flatten_vars_size + ( (x) - flatten_vars ) )
 
        .section ".prefix.lib", "awx", @progbits
        .code16
-pm_call:
-       /* Preserve registers, flags, and RM return point */
+flatten_real_mode:
+       /* Preserve registers and flags, allocate local variable block */
        pushw   %bp
        movw    %sp, %bp
-       subw    $pm_call_vars_size, %sp
+       subw    $flatten_vars_size, %sp
        andw    $0xfff0, %sp
        pushfl
+       pushl   %eax
+       pushl   %edi
+       pushw   %si
+       pushw   %cx
        pushw   %gs
        pushw   %fs
        pushw   %es
        pushw   %ds
        pushw   %ss
-       pushw   %cs
-       pushw   $99f
 
-       /* Set up local variable block, and preserve GDT */
-       pushw   %cx
-       pushw   %si
-       pushw   %di
+       /* Fill local variable block and preserve GDT */
        pushw   %ss
        popw    %es
-       movw    $pm_call_vars, %si
-       leaw    PM_CALL_VAR(pm_call_vars)(%bp), %di
-       movw    $pm_call_vars_size, %cx
+       movw    $flatten_vars, %si
+       leaw    FLATTEN_VAR(flatten_vars)(%bp), %di
+       movw    $flatten_vars_size, %cx
        cs rep movsb
-       popw    %di
-       popw    %si
-       popw    %cx
-       sgdt    PM_CALL_VAR(pm_saved_gdt)(%bp)
+       sgdt    FLATTEN_VAR(flatten_saved_gdt)(%bp)
 
        /* Set up GDT bases */
-       pushl   %eax
-       pushl   %edi
        xorl    %eax, %eax
        movw    %ss, %ax
        shll    $4, %eax
        movzwl  %bp, %edi
-       addr32 leal PM_CALL_VAR(gdt)(%eax, %edi), %eax
-       movl    %eax, PM_CALL_VAR(gdt_base)(%bp)
+       addr32 leal FLATTEN_VAR(flatten_gdt)(%eax, %edi), %eax
+       movl    %eax, FLATTEN_VAR(flatten_gdt_base)(%bp)
        movw    %cs, %ax
-       movw    $PM_CALL_VAR(pm_cs), %di
+       movw    $FLATTEN_VAR(flatten_cs), %di
        call    set_seg_base
        movw    %ss, %ax
-       movw    $PM_CALL_VAR(pm_ss), %di
+       movw    $FLATTEN_VAR(flatten_ss), %di
        call    set_seg_base
-       popl    %edi
-       popl    %eax
 
-       /* Switch CPU to protected mode and load up segment registers */
-       pushl   %eax
+       /* Switch temporarily to protected mode and set segment registers */
+       pushw   %cs
+       pushw   $2f
        cli
-       data32 lgdt PM_CALL_VAR(gdt)(%bp)
+       data32 lgdt FLATTEN_VAR(flatten_gdt)(%bp)
        movl    %cr0, %eax
        orb     $CR0_PE, %al
        movl    %eax, %cr0
-       ljmp    $PM_CS, $1f
-1:     movw    $PM_SS, %ax
+       ljmp    $FLAT_CS, $1f
+1:     movw    $FLAT_SS, %ax
        movw    %ax, %ss
-       movw    $PM_DS, %ax
-       movw    %ax, %ds
-       movw    %ax, %es
-       movw    %ax, %fs
-       movw    %ax, %gs
-       popl    %eax
-
-       /* Call PM routine */
-       call    *%ax
-
-       /* Set real-mode segment limits on %ds, %es, %fs and %gs */
-       movw    %ss, %ax
        movw    %ax, %ds
        movw    %ax, %es
        movw    %ax, %fs
        movw    %ax, %gs
-
-       /* Return CPU to real mode */
        movl    %cr0, %eax
        andb    $0!CR0_PE, %al
        movl    %eax, %cr0
+       lret
+2:     /* lret will ljmp to here */
 
-       /* Restore registers and flags */
-       lret    /* will ljmp to 99f */
-99:    popw    %ss
+       /* Restore GDT, registers and flags */
+       data32 lgdt FLATTEN_VAR(flatten_saved_gdt)(%bp)
+       popw    %ss
        popw    %ds
        popw    %es
        popw    %fs
        popw    %gs
-       data32 lgdt PM_CALL_VAR(pm_saved_gdt)(%bp)
+       popw    %cx
+       popw    %si
+       popl    %edi
+       popl    %eax
        popfl
        movw    %bp, %sp
        popw    %bp
        ret
-       .size pm_call, . - pm_call
+       .size flatten_real_mode, . - flatten_real_mode
 
 set_seg_base:
        rolw    $4, %ax
@@ -396,7 +368,7 @@ set_seg_base:
 #endif /* KEEP_IT_REAL */
 
 /****************************************************************************
- * copy_bytes (real-mode or 16-bit protected-mode near call)
+ * copy_bytes
  *
  * Copy bytes
  *
@@ -411,6 +383,7 @@ set_seg_base:
  *   None
  ****************************************************************************
  */
+#if ! COMPRESS
        .section ".prefix.lib", "awx", @progbits
        .code16
 copy_bytes:
@@ -419,9 +392,10 @@ copy_bytes:
        popl %ecx
        ret
        .size copy_bytes, . - copy_bytes
+#endif /* COMPRESS */
 
 /****************************************************************************
- * install_block (real-mode near call)
+ * install_block
  *
  * Install block to specified address
  *
@@ -439,39 +413,22 @@ copy_bytes:
        .section ".prefix.lib", "awx", @progbits
        .code16
 install_block:
-       
-#ifdef KEEP_IT_REAL
-
        /* Preserve registers */
        pushw   %ds
        pushw   %es
        pushl   %ecx
        pushl   %edi
        
-       /* Convert %esi and %edi to segment registers */
+       /* Convert %esi and %edi to %ds:esi and %es:edi */
        shrl    $4, %esi
        movw    %si, %ds
        xorw    %si, %si
+       shll    $4, %esi
        shrl    $4, %edi
        movw    %di, %es
        xorw    %di, %di
+       shll    $4, %edi
 
-#else /* KEEP_IT_REAL */
-
-       /* Call self in protected mode */
-       pushw   %ax
-       movw    $1f, %ax
-       call    pm_call
-       popw    %ax
-       ret
-1:
-       /* Preserve registers */
-       pushl   %ecx
-       pushl   %edi
-       
-#endif /* KEEP_IT_REAL */
-
-       
 #if COMPRESS
        /* Decompress source to destination */
        call    decompress16
@@ -492,33 +449,22 @@ install_block:
        addl    $0xf, %esi
        andl    $~0xf, %esi
 
-
-#ifdef KEEP_IT_REAL
-
        /* Convert %ds:esi back to a physical address */
-       movzwl  %ds, %cx
+       xorl    %ecx, %ecx
+       movw    %ds, %cx
        shll    $4, %ecx
        addl    %ecx, %esi
 
-       /* Restore registers */
+       /* Restore registers and return */
        popl    %edi
        popl    %ecx
        popw    %es
        popw    %ds
-
-#else /* KEEP_IT_REAL */
-
-       /* Restore registers */
-       popl    %edi
-       popl    %ecx
-
-#endif
-
        ret
        .size install_block, . - install_block
-       
+
 /****************************************************************************
- * alloc_basemem (real-mode near call)
+ * alloc_basemem
  *
  * Allocate space for .text16 and .data16 from top of base memory.
  * Memory is allocated using the BIOS free base memory counter at
@@ -568,7 +514,7 @@ alloc_basemem:
        .size alloc_basemem, . - alloc_basemem
 
 /****************************************************************************
- * free_basemem (real-mode near call)
+ * free_basemem
  *
  * Free space allocated with alloc_basemem.
  *
@@ -620,7 +566,7 @@ hooked_bios_interrupts:
        .size   hooked_bios_interrupts, . - hooked_bios_interrupts
 
 /****************************************************************************
- * install (real-mode near call)
+ * install
  *
  * Install all text and data segments.
  *
@@ -655,7 +601,7 @@ install:
        .size install, . - install
 
 /****************************************************************************
- * install_prealloc (real-mode near call)
+ * install_prealloc
  *
  * Install all text and data segments.
  *
@@ -680,6 +626,11 @@ install_prealloc:
        /* Sanity: clear the direction flag asap */
        cld
 
+#ifndef KEEP_IT_REAL
+       /* Flatten real mode */
+       call    flatten_real_mode
+#endif
+
        /* Calculate physical address of payload (i.e. first source) */
        testl   %esi, %esi
        jnz     1f
@@ -732,16 +683,15 @@ install_prealloc:
 
        /* Copy code to new location */
        pushl   %edi
-       pushw   %ax
-       movw    $copy_bytes, %ax
-       call    pm_call
-       popw    %ax
+       xorw    %ax, %ax
+       movw    %ax, %es
+       es rep addr32 movsb
        popl    %edi
 
        /* Initialise librm at new location */
        lcall   *init_librm_vector
-
 #endif
+
        /* Restore registers */
        popw    %es
        popw    %ds
@@ -749,7 +699,9 @@ install_prealloc:
        ret
        .size install_prealloc, . - install_prealloc
 
-       /* Vectors for far calls to .text16 functions */
+       /* Vectors for far calls to .text16 functions.  Must be in
+        * .data16, since .prefix may not be writable.
+        */
        .section ".data16", "aw", @progbits
 #ifdef KEEP_IT_REAL
 init_libkir_vector:
@@ -768,7 +720,7 @@ prot_call_vector:
 #endif
 
 /****************************************************************************
- * uninstall (real-mode near call)
+ * uninstall
  *
  * Uninstall all text and data segments.
  *