* Re: kernel loading question
2005-12-05 14:18 ` linux-os (Dick Johnson)
@ 2005-12-05 15:54 ` linux-os (Dick Johnson)
0 siblings, 0 replies; 4+ messages in thread
From: linux-os (Dick Johnson) @ 2005-12-05 15:54 UTC (permalink / raw)
To: Daniel Bonekeeper; +Cc: Linux kernel
[-- Attachment #1: Type: text/plain, Size: 6948 bytes --]
On Mon, 5 Dec 2005, linux-os (Dick Johnson) wrote:
>
> On Sat, 3 Dec 2005, Daniel Bonekeeper wrote:
>
>> Well, this probably should not be posted here, but who knows ? =)
>>
>> Well, on arch/i386/boot/compressed/head.S line 65 we call
>> decompress_kernel that decompresses the kernel starting at $0x100000.
>> If we were loaded high, we first move the code back to $0x1000 before
>> ljmp'ing it.
>>
>> My question is: why, when loaded high, we care to disable interrupts
>> (cli # make sure we don't get interrupted) while otherwise, we don't
>> do that ? in both cases, aren't we supposed to disable cli (as they
>> can happen in both situations) ?
>>
>> Since currently, the kernel boots fine for everybody in the world, I
>> know that this is not a bug... but why don't disable interruptions on
>> both cases ?
>>
>> Thanks !
>
> The first thing to do when looking at assembly language files
> is to find what symbols are global (.globl or .global). Unlike
> 'C' the stuff doesn't default to being visible from other
> files. Given that, you will see that code executes from
> startup_32, the only way "in" since it's the only global
> symbol. The second instruction is 'cli', which will disable
> interrupts on the current processor.
>
> Now, later on in the code is the sequence:
> pushl $0
> popfl
>
> ... which will further clear any flags so the interrupt-enable
> flag is truly disabled there, also. Later on, there are redundant
> 'cld' instructions as well as an additional 'cli' instruction.
>
> These probably occurred because the code was assembled from
> various pieces and nobody bothered to review it for such
> redundancy.
>
> Getting rid of the extra instructions will save 3 or 4 bytes
> of code. Since this code executes only once, it's certainly
> not in the "fast-path" so nobody has bothered to clean it.
> If you would like to clean up the code and submit a patch
> it probably would be accepted.
>
> I will test any patch you may want to produce.
>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.13.4 on an i686 machine (5589.44 BogoMips).
> Warning : 98.36% of all statistics are fiction.
Here is a patch that works both on the normal Pentium and the
ELAN which is a '486'. It hasn't been tested on __everything__
because I don't have a '386. Nevertheless, it should work
everywhere and it contains a few cleanups as well. Just in
case the mailer screws it up, it is attached as a plain
text file as well.
Signed Off By: Richard B. Johnson rjohnson@analogic.com
--------
--- linux-2.6.13.4/arch/i386/boot/compressed/head.S.orig 2005-12-05 09:23:22.000000000 -0500
+++ linux-2.6.13.4/arch/i386/boot/compressed/head.S 2005-12-05 10:39:56.000000000 -0500
@@ -18,9 +18,21 @@
* mode.
*/
-/*
- * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
- */
+#
+# High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
+#
+# Dec 5, 2005 rjohnson@analogic.com
+# Removed most of the old-style 'C' comment format and replaced with
+# GAS comment delimiters.
+#
+# Dec 5, 2005 rjohnson@analogic.com
+# Removed many redundant flag operations. Adjusted offset of
+# the move_routine_start to a 16-byte boundary so it is truly
+# position-independent.
+#
+# Dec 5, 2005 rjohnson@analogic.com
+# Fixed A20 detection code.
+#
.text
#include <linux/linkage.h>
@@ -30,41 +42,38 @@
.globl startup_32
startup_32:
- cld
cli
- movl $(__BOOT_DS),%eax
- movl %eax,%ds
+ movl $(__BOOT_DS),%eax # Linear address-space
+ movl %eax,%ds # Set segments
movl %eax,%es
movl %eax,%fs
movl %eax,%gs
- lss stack_start,%esp
- xorl %eax,%eax
-1: incl %eax # check that A20 really IS enabled
- movl %eax,0x000000 # loop forever if it isn't
- cmpl %eax,0x100000
- je 1b
-
-/*
- * Initialize eflags. Some BIOS's leave bits like NT set. This would
- * confuse the debugger if this code is traced.
- * XXX - best to initialize before switching to protected mode.
- */
- pushl $0
- popfl
-/*
- * Clear BSS
- */
+ lss stack_start,%esp # Set stack
+ xorl %eax,%eax # Clear
+ pushl %eax
+ popfl # Clear all flags
+#
+# Check for at least 20-bit addressing, (A20 is enabled).
+#
+ movl $__PHYSICAL_START, %ebx
+ movl (%ebx), %eax # Save whatever is there
+ movl $0xfeedface, (%ebx) # Set magic
+1: cmpl $0xfeedface, (%ebx) # See if 32-bits
+ jnz 1b # Hang forever if not
+ movl %eax, (%ebx) # Put the code back
+#
+# Clear BSS
+#
xorl %eax,%eax
movl $_edata,%edi
movl $_end,%ecx
subl %edi,%ecx
- cld
rep
stosb
-/*
- * Do the decompression, and jump to the new kernel..
- */
+#
+# Do the decompression, and jump to the new kernel..
+#
subl $16,%esp # place for structure on the stack
movl %esp,%eax
pushl %esi # real mode pointer as second arg
@@ -77,20 +86,18 @@
xorl %ebx,%ebx
ljmp $(__BOOT_CS), $__PHYSICAL_START
-/*
- * We come here, if we were loaded high.
- * We need to move the move-in-place routine down to 0x1000
- * and then start it with the buffer addresses in registers,
- * which we got from the stack.
- */
-3:
- movl $move_routine_start,%esi
+#
+# We come here, if we were loaded high.
+# We need to move the move-in-place routine down to 0x1000
+# and then start it with the buffer addresses in registers
+# which we got from the stack.
+#
+3: movl $move_routine_start,%esi
movl $0x1000,%edi
movl $move_routine_end,%ecx
subl %esi,%ecx
addl $3,%ecx
shrl $2,%ecx
- cld
rep
movsl
@@ -101,13 +108,13 @@
popl %edx # high_buffer_start
popl %eax # hcount
movl $__PHYSICAL_START,%edi
- cli # make sure we don't get interrupted
ljmp $(__BOOT_CS), $0x1000 # and jump to the move routine
-/*
- * Routine (template) for moving the decompressed kernel in place,
- * if we were high loaded. This _must_ PIC-code !
- */
+#
+# Routine (template) for moving the decompressed kernel in place,
+# when we are high loaded. This _must_ be position-independent code!
+#
+.align 0x10
move_routine_start:
movl %ecx,%ebp
shrl $2,%ecx
-----------
Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.44 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
****************************************************************
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Thank you.
[-- Attachment #2: head.patch --]
[-- Type: TEXT/PLAIN, Size: 3568 bytes --]
--- linux-2.6.13.4/arch/i386/boot/compressed/head.S.orig 2005-12-05 09:23:22.000000000 -0500
+++ linux-2.6.13.4/arch/i386/boot/compressed/head.S 2005-12-05 10:39:56.000000000 -0500
@@ -18,9 +18,21 @@
* mode.
*/
-/*
- * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
- */
+#
+# High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
+#
+# Dec 5, 2005 rjohnson@analogic.com
+# Removed most of the old-style 'C' comment format and replaced with
+# GAS comment delimiters.
+#
+# Dec 5, 2005 rjohnson@analogic.com
+# Removed many redundant flag operations. Adjusted offset of
+# the move_routine_start to a 16-byte boundary so it is truly
+# position-independent.
+#
+# Dec 5, 2005 rjohnson@analogic.com
+# Fixed A20 detection code.
+#
.text
#include <linux/linkage.h>
@@ -30,41 +42,38 @@
.globl startup_32
startup_32:
- cld
cli
- movl $(__BOOT_DS),%eax
- movl %eax,%ds
+ movl $(__BOOT_DS),%eax # Linear address-space
+ movl %eax,%ds # Set segments
movl %eax,%es
movl %eax,%fs
movl %eax,%gs
- lss stack_start,%esp
- xorl %eax,%eax
-1: incl %eax # check that A20 really IS enabled
- movl %eax,0x000000 # loop forever if it isn't
- cmpl %eax,0x100000
- je 1b
-
-/*
- * Initialize eflags. Some BIOS's leave bits like NT set. This would
- * confuse the debugger if this code is traced.
- * XXX - best to initialize before switching to protected mode.
- */
- pushl $0
- popfl
-/*
- * Clear BSS
- */
+ lss stack_start,%esp # Set stack
+ xorl %eax,%eax # Clear
+ pushl %eax
+ popfl # Clear all flags
+#
+# Check for at least 20-bit addressing, (A20 is enabled).
+#
+ movl $__PHYSICAL_START, %ebx
+ movl (%ebx), %eax # Save whatever is there
+ movl $0xfeedface, (%ebx) # Set magic
+1: cmpl $0xfeedface, (%ebx) # See if 32-bits
+ jnz 1b # Hang forever if not
+ movl %eax, (%ebx) # Put the code back
+#
+# Clear BSS
+#
xorl %eax,%eax
movl $_edata,%edi
movl $_end,%ecx
subl %edi,%ecx
- cld
rep
stosb
-/*
- * Do the decompression, and jump to the new kernel..
- */
+#
+# Do the decompression, and jump to the new kernel..
+#
subl $16,%esp # place for structure on the stack
movl %esp,%eax
pushl %esi # real mode pointer as second arg
@@ -77,20 +86,18 @@
xorl %ebx,%ebx
ljmp $(__BOOT_CS), $__PHYSICAL_START
-/*
- * We come here, if we were loaded high.
- * We need to move the move-in-place routine down to 0x1000
- * and then start it with the buffer addresses in registers,
- * which we got from the stack.
- */
-3:
- movl $move_routine_start,%esi
+#
+# We come here, if we were loaded high.
+# We need to move the move-in-place routine down to 0x1000
+# and then start it with the buffer addresses in registers
+# which we got from the stack.
+#
+3: movl $move_routine_start,%esi
movl $0x1000,%edi
movl $move_routine_end,%ecx
subl %esi,%ecx
addl $3,%ecx
shrl $2,%ecx
- cld
rep
movsl
@@ -101,13 +108,13 @@
popl %edx # high_buffer_start
popl %eax # hcount
movl $__PHYSICAL_START,%edi
- cli # make sure we don't get interrupted
ljmp $(__BOOT_CS), $0x1000 # and jump to the move routine
-/*
- * Routine (template) for moving the decompressed kernel in place,
- * if we were high loaded. This _must_ PIC-code !
- */
+#
+# Routine (template) for moving the decompressed kernel in place,
+# when we are high loaded. This _must_ be position-independent code!
+#
+.align 0x10
move_routine_start:
movl %ecx,%ebp
shrl $2,%ecx
^ permalink raw reply [flat|nested] 4+ messages in thread