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Byron Stanoszek

2007-Jan-04 19:17 UTC

[syslinux] Automatically choose between 32-bit and 64-bit kernel

I've been using syslinux on my own Linux recovery CDs for almost a year now,
and in that time I've found it useful to be able to automatically load a
64-bit-capable kernel if the CPU supports it.

I'd like to share my efforts and submit this patch for syslinux-3.31. It
adds a
new config file keyword, 'default64', which overrides the default image
if a
64-bit CPU is detected. This allows your config file to have something like
this in order to automatically choose between a 32-bit and 64-bit kernel:

      default linux
      default64 linux64
      prompt 0

      label linux
        kernel vmlinuz
        append root=/dev/sr0

      label linux64
        kernel vmlinuz-64bit
        append root=/dev/sr0

Alternatively, this patch can be grabbed from:
http://www.winds.org/pub/linux/syslinux/syslinux-3.31-64bit.patch

It should be 386-safe, but I don't have a 386 to test it out with.

Best regards,
   -Byron


diff -ur syslinux-3.31.orig/cpuinit.inc syslinux-3.31/cpuinit.inc
--- syslinux-3.31.orig/cpuinit.inc	2006-09-26 00:52:22.000000000 -0400
+++ syslinux-3.31/cpuinit.inc	2007-01-03 14:41:27.000000000 -0500
@@ -48,12 +48,10 @@
  		 rep movsd

  ;
-; Check if we're 386 (as opposed to 486+); if so we need to blank out
-; the WBINVD instruction
+; Determine if we're running on a 64-bit CPU
  ;
-; We check for 486 by setting EFLAGS.AC
+; First, check if we're 386. If so, we need to blank out the WBINVD 
instruction
  ;
-%if DO_WBINVD
  		 pushfd				; Save the good flags
  		 pushfd
  		 pop eax
@@ -69,6 +67,47 @@
  ;
  ;  386 - Looks like we better blot out the WBINVD instruction
  ; 
+%if DO_WBINVD
  		mov byte [try_wbinvd],0c3h		; Near RET
-is_486:
  %endif	; DO_WBINVD
+		jmp is_32bit
+is_486:
+;
+; Check if this CPU supports the CPUID command
+;
+		pushfd				; Save the flags again
+		pushfd
+		pop eax
+		mov ebx,eax
+		xor eax,(1 << 21)		; CPUID bit
+		push eax
+		popfd
+		pushfd
+		pop eax
+		popfd				; Restore the original flags
+		xor eax,ebx
+		jz is_32bit
+;
+; Now check for the 64-bit flag in the CPU features byte ($0000_0001, edx)
+; This is bit 30 for Intel CPUs, and bit 29 for AMD CPUs
+;
+		mov eax, 00000000h		; Find last Intel cpuid #
+		cpuid
+		cmp eax, 00000000h
+		je test_amd
+		mov eax, 00000001h		; Read Intel CPU flags
+		cpuid
+		bt edx, 30			; 64-bit if bit 30 is set
+		jc is_64bit
+
+test_amd:	mov eax, 80000000h		; Find last AMD cpuid #
+		cpuid
+		cmp eax, 80000000h
+		jbe is_32bit
+		mov eax, 80000001h		; Read AMD CPU flags
+		cpuid
+		bt edx, 29			; 64-bit if bit 29 is set
+		jnc is_32bit
+
+is_64bit:	mov byte [Is64Bit],1		; Flag that we're 64-bit
+is_32bit:
diff -ur syslinux-3.31.orig/keywords syslinux-3.31/keywords
--- syslinux-3.31.orig/keywords	2006-09-26 00:52:22.000000000 -0400
+++ syslinux-3.31/keywords	2007-01-03 14:02:22.000000000 -0500
@@ -1,6 +1,7 @@
   menu
   append
   default
+default64
   display
   font
   implicit
diff -ur syslinux-3.31.orig/keywords.inc syslinux-3.31/keywords.inc
--- syslinux-3.31.orig/keywords.inc	2006-09-26 00:52:22.000000000 -0400
+++ syslinux-3.31/keywords.inc	2007-01-03 14:02:38.000000000 -0500
@@ -46,6 +46,7 @@
  		 keyword menu,	   pc_comment
  		 keyword append,    pc_append
  		 keyword default,   pc_default
+		keyword default64, pc_default64
  		 keyword display,   pc_filecmd,  get_msg_file
  		 keyword font,      pc_filecmd,  loadfont
  		 keyword implicit,  pc_setint16, AllowImplicit
diff -ur syslinux-3.31.orig/kwdhash.gen syslinux-3.31/kwdhash.gen
--- syslinux-3.31.orig/kwdhash.gen	2006-09-26 00:52:27.000000000 -0400
+++ syslinux-3.31/kwdhash.gen	2007-01-03 15:17:26.000000000 -0500
@@ -1,6 +1,7 @@
   hash_menu               equ 0x003719b5
   hash_append             equ 0xc53999a4
   hash_default            equ 0xcc5159ed
+hash_default64          equ 0x4567b1c5
   hash_display            equ 0xd509bc40
   hash_font               equ 0x0032b1b4
   hash_implicit           equ 0xa6f50207
diff -ur syslinux-3.31.orig/parseconfig.inc syslinux-3.31/parseconfig.inc
--- syslinux-3.31.orig/parseconfig.inc	2006-09-26 00:52:22.000000000 -0400
+++ syslinux-3.31/parseconfig.inc	2007-01-03 16:34:19.000000000 -0500
@@ -20,7 +20,20 @@
  ;
  ;  "default" command
  ; 
-pc_default:	mov di,default_cmd
+pc_default:	cmp byte [HasDefault64],0	; Check if we accepted 
'default64'
+		ja pc_getline			; If so, do nothing
+		mov di,default_cmd
+		call getline
+		mov byte [di-1],0		; null-terminate
+		ret
+
+;
+; "default64" command
+;
+pc_default64:	cmp byte [Is64Bit],0		; Make sure cpu is 64-bit
+		je pc_getline
+		mov byte [HasDefault64],1	; Note that we saw a default64
+		mov di,default_cmd
  		 call getline
  		 mov byte [di-1],0		; null-terminate
  		 ret
@@ -389,6 +402,8 @@
   SerialPort	dw 0			; Serial port base (or 0 for no serial
   port)
   VKernelBytes	dw 0			; Number of bytes used by vkernels
   VKernel		db 0			; Have we seen any "label"
   statements?
+Is64Bit		db 0			; Is this CPU 64-bit?
+HasDefault64	db 0			; We've seen a 'default64' statement

  		 section .latebss
                   alignb 4		; For the good of REP MOVSD


--
Byron Stanoszek                         Ph: (330) 644-3059
Systems Programmer                      Fax: (330) 644-8110
Commercial Timesharing Inc.             Email: byron at comtime.com

H. Peter Anvin

2007-Jan-04 19:33 UTC

head link

[syslinux] Automatically choose between 32-bit and 64-bit kernel

Byron Stanoszek wrote:> I've been using syslinux on my own Linux recovery CDs for almost a year
now,
> and in that time I've found it useful to be able to automatically load
a
> 64-bit-capable kernel if the CPU supports it.
> 
> I'd like to share my efforts and submit this patch for syslinux-3.31.
It adds a
> new config file keyword, 'default64', which overrides the default
image if a
> 64-bit CPU is detected. This allows your config file to have something like
> this in order to automatically choose between a 32-bit and 64-bit kernel:
> 
>       default linux
>       default64 linux64
>       prompt 0
> 
>       label linux
>         kernel vmlinuz
>         append root=/dev/sr0
> 
>       label linux64
>         kernel vmlinuz-64bit
>         append root=/dev/sr0
> 
> Alternatively, this patch can be grabbed from:
> http://www.winds.org/pub/linux/syslinux/syslinux-3.31-64bit.patch
> 
I really don't want to put this kind of stuff in the assembly code. 
There are arbitrary complex rules one may want to have for kernel 
selection, and that stuff belongs in a com32 module.

	-hpa

Erwan Velu

2007-Jan-05 10:59 UTC

head link

[syslinux] Automatically choose between 32-bit and 64-bit kernel

Byron Stanoszek wrote:> I've been using syslinux on my own Linux recovery CDs for almost a year
now,
> and in that time I've found it useful to be able to automatically load
a
> 64-bit-capable kernel if the CPU supports it.
>
> I'd like to share my efforts and submit this patch for syslinux-3.31.
It adds a
> new config file keyword, 'default64', which overrides the default
image if a
> 64-bit CPU is detected. This allows your config file to have something like
> this in order to automatically choose between a 32-bit and 64-bit kernel:
>   I've been starting some stuff on the hardware detection. Some com32 
modules are able to detect the hardware capabilities.
The x86_64 capability is one of those. Now we need to define a kind of 
script language than can help people to defines rules
to match some hardware capability and some syslinux configuration.

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