[romprefix] Inhibit the use of relocation during POST

It is common for system memory maps to be grotesquely unreliable
during POST.  Many sanity checks have been added to the memory map
reading code, but these do not catch all problems.

Skip relocation entirely if called during POST.  This should avoid the
problems typically encountered, at the cost of slightly disrupting the
memory map of an operating system booted via gPXE when gPXE was
entered during POST.  Since this is a very rare special case (used,
for example, when reflashing an experimental ROM that would otherwise
prevent the system from completing POST), this is an acceptable cost.

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 f5f6691..eeed0ad 100644 (file)
--- a/src/arch/i386/prefix/libprefix.S
+++ b/src/arch/i386/prefix/libprefix.S
@@ -443,6 +443,8 @@ install:
        xorl    %esi, %esi
        /* Image destination = default */
        xorl    %edi, %edi
+       /* Allow relocation */
+       clc
        /* Install text and data segments */
        call    install_prealloc
        /* Restore registers and return */
@@ -461,6 +463,7 @@ install:
  *   %bx  : .data16 segment address
  *   %esi : Image source physical address (or zero for %cs:0000)
  *   %edi : Decompression temporary area physical address (or zero for default)
+ *   CF set : Avoid relocating to top of memory
  * Corrupts:
  *   none
  ****************************************************************************
@@ -473,9 +476,8 @@ install_prealloc:
        pushal
        pushw   %ds
        pushw   %es
-
-       /* Sanity: clear the direction flag asap */
-       cld
+       cld                     /* Sanity: clear the direction flag asap */
+       pushfw
 
        /* Copy decompression temporary area physical address to %ebp */
        movl    %edi, %ebp
@@ -570,6 +572,11 @@ a20_death_message:
        movl    %ebp, %edi
        lcall   *init_librm_vector
 
+       /* Skip relocation if CF was set on entry */
+       popfw
+       pushfw
+       jc      skip_relocate
+
        /* Call relocate() to determine target address for relocation.
         * relocate() will return with %esi, %edi and %ecx set up
         * ready for the copy to the new location.
@@ -588,9 +595,11 @@ a20_death_message:
 
        /* Initialise librm at new location */
        lcall   *init_librm_vector
+skip_relocate:
 #endif
 
        /* Restore registers */
+       popfw
        popw    %es
        popw    %ds
        popal

diff --git a/src/arch/i386/prefix/romprefix.S b/src/arch/i386/prefix/romprefix.S
index 7a1b0e7..8d37baa 100644 (file)
--- a/src/arch/i386/prefix/romprefix.S
+++ b/src/arch/i386/prefix/romprefix.S
@@ -407,6 +407,7 @@ no_pmm:
         * picked up by the initial shell prompt, and we will drop
         * into a shell.
         */
+       stc                     /* Inhibit relocation */
        pushw   %cs
        call    exec
 2:
@@ -597,6 +598,7 @@ bbs_version:
  * Called by the PnP BIOS when it wants to boot us.
  */
 bev_entry:
+       clc                     /* Allow relocation */
        pushw   %cs
        call    exec
        lret
@@ -631,6 +633,7 @@ int19_entry:
        /* Leave keypress in buffer and start gPXE.  The keypress will
         * cause the usual initial Ctrl-B prompt to be skipped.
         */
+       clc                     /* Allow relocation */
        pushw   %cs
        call    exec
 1:     /* Try to call original INT 19 vector */
@@ -662,6 +665,9 @@ exec:       /* Set %ds = %cs */
        pushw   %cs
        popw    %ds
 
+       /* Preserve state of CF */
+       lahf
+
        /* Print message as soon as possible */
        movw    $prodstr, %si
        xorw    %di, %di
@@ -675,14 +681,17 @@ exec:     /* Set %ds = %cs */
        movw    %sp, %bp
 
        /* Obtain a reasonably-sized temporary stack */
-       xorw    %ax, %ax
-       movw    %ax, %ss
+       xorw    %bx, %bx
+       movw    %bx, %ss
        movw    $0x7c00, %sp
 
        /* Install gPXE */
+       sahf
+       pushfw
+       call    alloc_basemem
+       popfw
        movl    image_source, %esi
        movl    decompress_to, %edi
-       call    alloc_basemem
        call    install_prealloc
 
        /* Print message indicating successful installation */

diff --git a/src/arch/i386/prefix/undiloader.S b/src/arch/i386/prefix/undiloader.S
index 36c1bef..6fab3a3 100644 (file)
--- a/src/arch/i386/prefix/undiloader.S
+++ b/src/arch/i386/prefix/undiloader.S
@@ -28,6 +28,7 @@ undiloader:
        movw    %es:14(%di), %ax
        movl    image_source, %esi
        movl    decompress_to, %edi
+       clc                     /* Allow relocation */
        call    install_prealloc
        popw    %di
        /* Call UNDI loader C code */