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 !
--
# (perl -e "while (1) { print "\x90"; }") | dd of=/dev/evil
On Fri, Dec 02, 2005 at 09:34:26PM -0800, 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 ?
Agreed it's strange. I believe that since very few people load the
kernel in low memory nowadays given its size, it's possible that the
missing CLI never gets noticed, and I suspect that the very first
instructions after the jump are a CLI anyway.
Regards,
Willy
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.
.
****************************************************************
The information transmitted in this message is confidential and may be privileged. Any review, retransmission, dissemination, or other use of this information by persons or entities other than the intended recipient is prohibited. If you are not the intended recipient, please notify Analogic Corporation immediately - by replying to this message or by sending an email to [email protected] - and destroy all copies of this information, including any attachments, without reading or disclosing them.
Thank you.
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 [email protected]
--------
--- 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 [email protected]
+# Removed most of the old-style 'C' comment format and replaced with
+# GAS comment delimiters.
+#
+# Dec 5, 2005 [email protected]
+# 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 [email protected]
+# 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.
.
****************************************************************
The information transmitted in this message is confidential and may be privileged. Any review, retransmission, dissemination, or other use of this information by persons or entities other than the intended recipient is prohibited. If you are not the intended recipient, please notify Analogic Corporation immediately - by replying to this message or by sending an email to [email protected] - and destroy all copies of this information, including any attachments, without reading or disclosing them.
Thank you.