Regarding enable paging code and swapper_pg_dir.

Regarding enable paging code and swapper_pg_dir.

[mindentropy]

Hi All,

  In the code http://lxr.linux.no/#linux+v2.6.12/arch/i386/kernel/head.S#L186 
I am not able to understand what's the reason for subtracting swapper_pg_dir 
with __PAGE_OFFSET(0xc0000000). i.e. movl $swapper_pg_dir-__PAGE_OFFSET 
Shouldn't it be movl $swapper_pg_dir, %eax?

Thanks.

Regarding enable paging code and swapper_pg_dir.

[James Light]

On Sat, Apr 02, 2011 at 01:33:21PM +0530, mindentropy wrote:
> On Saturday 02 Apr 2011 2:49:21 am you wrote:
> > On Sat, Apr 02, 2011 at 01:37:12AM +0530, mindentropy wrote:
> > > Hi All,
> > > 
> > >   In the code
> > >   http://lxr.linux.no/#linux+v2.6.12/arch/i386/kernel/head.S#L186
> > > 
> > > I am not able to understand what's the reason for subtracting
> > > swapper_pg_dir with __PAGE_OFFSET(0xc0000000). i.e. movl
> > > $swapper_pg_dir-__PAGE_OFFSET Shouldn't it be movl $swapper_pg_dir,
> > > %eax?
> > > 
> > > Thanks.
> > > 
> > > _______________________________________________
> > > Kernelnewbies mailing list
> > 
> > I'm no expert here, but I think the reason that it is coded that way has to
> > do with the fact that the swapper_pg_dir is defined to be the value that
> > will be used once the CPU has switched to Protected Mode, but this asm
> > code in head.S is executing while the processes is in Real Mode.
> > 
> > The instruction that enables paging and protected mode addressing is on
> > line 190, I believe.
> > 
> >      -James
> 
> Yes but why the subtraction with __PAGE_OFFSET? 

To put the physical address into cr3 later.
$swapper_pg_dir is not the PHYSICAL address of the PGD.
So they have to subtract 0xc0000000 (kernel code segment base linear address)
from that the get the physical address as the logical address can not be used
yet because the paging unit isn't enabled yet.

Again, my understanding may be wrong. As I said before, I'm just learning this
as well.
     -James

Regarding enable paging code and swapper_pg_dir.

[James Light]

If you look at the 2.6.38 code for head.S, you may find the newer way it is
written to be a bit easier to read.

------------------------------------------------------------------------------
25 /* Physical address */
26 #define pa(X) ((X) - __PAGE_OFFSET)

...

345 /*
346 * Enable paging
347 */
348        movl $pa(initial_page_table), %eax
349        movl %eax,%cr3          /* set the page table pointer.. */
350        movl %cr0,%eax
351        orl  $X86_CR0_PG,%eax
352        movl %eax,%cr0          /* ..and set paging (PG) bit */
353        ljmp $__BOOT_CS,$1f     /* Clear prefetch and normalize %eip */

------------------------------------------------------------------------------

     -James

Regarding enable paging code and swapper_pg_dir.

[mindentropy]

On Saturday 02 Apr 2011 9:55:35 pm James Light wrote:
> 
> To put the physical address into cr3 later.
> $swapper_pg_dir is not the PHYSICAL address of the PGD.

Correct me if I am wrong but a mov $foo,%eax would move the address of foo  
variable to eax right? Or is there a macro somewhere where they are adding 
PAGE_OFFSET to the address so that the $swapper_pg_dir - __PAGE_OFFSET 
justifies?

Regarding enable paging code and swapper_pg_dir.

[James Light]

On Sat, Apr 02, 2011 at 11:18:19PM +0530, mindentropy wrote:
> On Saturday 02 Apr 2011 9:55:35 pm James Light wrote:
> > 
> > To put the physical address into cr3 later.
> > $swapper_pg_dir is not the PHYSICAL address of the PGD.
> 
> Correct me if I am wrong but a mov $foo,%eax would move the address of foo  
> variable to eax right? Or is there a macro somewhere where they are adding 
> PAGE_OFFSET to the address so that the $swapper_pg_dir - __PAGE_OFFSET 
> justifies?

Right, but it uses the logical address of foo which n the case of swapper_pg_dir
must be converted to a
physical address. When paging is not enabled, linear addresses are interpreted
as physical addressess. So it has to get this logical address only one step
farther into a linear address.

So, this "swapper_pg_dir" is a symbol in a section of code. That section of code
has an associated segment and this symbol lives at a particular location in that
section and thus in that segment, and that particular location, relative to the
beginning of the section is it's offset. Combine those two and
you have the logical address of swapper_pg_dir. 

The sigil "$" is the immediate
value sigil and the symbol $swapper_pg_dir thus uses the immediate value of
swapper_pg_dir. The immediate value of swapper_pg_dir is the value held at the
location that is logically labeled by the symbol swapper_pg_dir.

It may help to remind yourself that symbol names in assembly are very similar to
labels in assembly. They mark a location in the program. The program runs and
while running it only uses logical addresses. This particular code is loading a
linear (and w/out paging therefore physical) address. 
This is simply because of the design of the cr3 register and
paging in x86. 

>From http://www.intel.com/design/processor/manuals/253668.pdf 
Chapter 4: Paging (Page 1)
------------------------------------------------------------------------------
Software enables paging by using the MOV to CR0 instruction to set CR0.PG.
Before
doing so, software should ensure that control register CR3 contains the physical
address of the first paging structure that the processor will use for
linear-address
translation (see Section 4.2)

------------------------------------------------------------------------------


In linux2.1.66, the physical address is used directly.
Just for comparison:
------------------------------------------------------------------------------
58/*
59 * Setup paging (the tables are already set up, just switch them on)
60 */
611:
62        movl $0x101000,%eax
63        movl %eax,%cr3          /* set the page table pointer.. */
64        movl %cr0,%eax
65        orl $0x80000000,%eax
66        movl %eax,%cr0          /* ..and set paging (PG) bit */
67        jmp 1f                  /* flush the prefetch-queue */
------------------------------------------------------------------------------

If any of my own reasoning is wrong, I hope someone w/ more clue jumps in. ;)


     -James L

Regarding enable paging code and swapper_pg_dir.

[mindentropy]

On Sunday 03 Apr 2011 2:22:19 am James Light wrote:
> On Sat, Apr 02, 2011 at 11:18:19PM +0530, mindentropy wrote:
> > On Saturday 02 Apr 2011 9:55:35 pm James Light wrote:
> > > To put the physical address into cr3 later.
> > > $swapper_pg_dir is not the PHYSICAL address of the PGD.
> > 
> > Correct me if I am wrong but a mov $foo,%eax would move the address of
> > foo variable to eax right? Or is there a macro somewhere where they are
> > adding PAGE_OFFSET to the address so that the $swapper_pg_dir -
> > __PAGE_OFFSET justifies?
> 
> Right, but it uses the logical address of foo which n the case of
> swapper_pg_dir must be converted to a
> physical address. When paging is not enabled, linear addresses are
> interpreted as physical addressess.
> So it has to get this logical address
> only one step farther into a linear address.
> 

Can you please elaborate on what one step farther is? So why not use the 
swapper_pg_dir directly i.e. mov $swapper_pg_dir,cr0? In the absence of paging 
the segment:offset  is the physical address isn't it? i.e. the  $swapper_pg_dir 
should be equal to 0x10100 as per the code fragment below?

I am having difficulty in understanding the addressing calculations here and not 
the code,

>In linux2.1.66, the physical address is used directly.
>Just for comparison:
>------------------------------------------------------------------------------
>58/*
>59 * Setup paging (the tables are already set up, just switch them on)
>60 */
>611:
>62        movl $0x101000,%eax
>63        movl %eax,%cr3          /* set the page table pointer.. */
>64        movl %cr0,%eax
>65        orl $0x80000000,%eax
>66        movl %eax,%cr0          /* ..and set paging (PG) bit */
>67        jmp 1f                  /* flush the prefetch-queue */
>-----------------------------------------------------------------------------

Regarding enable paging code and swapper_pg_dir.

[mindentropy]

On Sunday 03 Apr 2011 2:22:19 am James Light wrote:
> On Sat, Apr 02, 2011 at 11:18:19PM +0530, mindentropy wrote:
> > On Saturday 02 Apr 2011 9:55:35 pm James Light wrote:
> > > To put the physical address into cr3 later.
> > > $swapper_pg_dir is not the PHYSICAL address of the PGD.
> > 
> > Correct me if I am wrong but a mov $foo,%eax would move the address of
> > foo variable to eax right? Or is there a macro somewhere where they are
> > adding PAGE_OFFSET to the address so that the $swapper_pg_dir -
> > __PAGE_OFFSET justifies?
> 
> Right, but it uses the logical address of foo which n the case of
> swapper_pg_dir must be converted to a
> physical address. When paging is not enabled, linear addresses are
> interpreted as physical addressess. So it has to get this logical address
> only one step farther into a linear address.
> 
> So, this "swapper_pg_dir" is a symbol in a section of code. That section of
> code has an associated segment and this symbol lives at a particular
> location in that section and thus in that segment, and that particular
> location, relative to the beginning of the section is it's offset. Combine
> those two and
> you have the logical address of swapper_pg_dir.
> 
Ok I think I got where the addresses are assigned. Its in the linker script 
vmlinux.lds. I got the precise answer here http://tldp.org/HOWTO/Linux-i386-
Boot-Code-HOWTO/kernel_head.html
I had completely forgotten about linker scripts. I was getting confused as the 
ENTRY(swapper_pg_dir) has ORG 0x10000 as the offset. Was struggling to find 
where the .text address was.