paging question

paging question

[Daniel Stodden]

hi.

i desperately hope this is not too stupid.

i'm trying to write a driver which depends on giving pci devices
access to somewhat larger amounts of pysical memory. let's say, a
megabyte of contiguous ram.

is it possible to resize such an area later on? i mean: is there some
mechanism available in the kernel to enlarge such a region even if the
area beyond it is already in use?

i understand that this is pretty impossible if some entity depends on
correct physical locations of the pages in question. but couldn't for
example userland memory be copied elsewhere and its new location
simply remapped?

regards,
dns

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Re: paging question

[Rik van Riel]

On Fri, 9 Feb 2001, Daniel Stodden wrote:

> i desperately hope this is not too stupid.

Only if the hardware is so stupid that you need this ;)

> i'm trying to write a driver which depends on giving pci devices
> access to somewhat larger amounts of pysical memory. let's say, a
> megabyte of contiguous ram.
> 
> is it possible to resize such an area later on? i mean: is there some
> mechanism available in the kernel to enlarge such a region even if the
> area beyond it is already in use?
> 
> i understand that this is pretty impossible if some entity depends on
> correct physical locations of the pages in question. but couldn't for
> example userland memory be copied elsewhere and its new location
> simply remapped?

Currently Linux doesn't have any way to figure out which
programs are using a certain physical memory location, so
we cannot efficiently unmap the memory.

And even if we could ... what if the memory is in use by
the kernel itself and there are pointers to those kernel
addresses somewhere else in the kernel ?    These pages
would be unfreeable anyway...

regards,

Rik
--
Linux MM bugzilla: http://linux-mm.org/bugzilla.shtml

Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

		http://www.surriel.com/
http://www.conectiva.com/	http://distro.conectiva.com/

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Re: paging question

[Jeff Hartmann]

Daniel Stodden wrote:

> hi.
> 
> i desperately hope this is not too stupid.
> 
> i'm trying to write a driver which depends on giving pci devices
> access to somewhat larger amounts of pysical memory. let's say, a
> megabyte of contiguous ram.

Your unlikely to get 1 MB of contigous ram unless you grab it very early 
in the boot process.  This means your driver needs to be built into the 
kernel, it can't be a module.

> 
> 
> is it possible to resize such an area later on? i mean: is there some
> mechanism available in the kernel to enlarge such a region even if the
> area beyond it is already in use?

No.

> 
> 
> i understand that this is pretty impossible if some entity depends on
> correct physical locations of the pages in question. but couldn't for
> example userland memory be copied elsewhere and its new location
> simply remapped?

If we had reverse page tables we could perhaps do this sort of 
remapping.  Currently there is no way to detrimine which physical page 
maps to which userland page without scanning every processes page 
tables.  There is also the possibility that the memory is used by the 
kernel, in which case your basically out of luck.

-Jeff

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paging question

[Jason Yan]

Hi,

I have a question about the code to enable  to initialize page 
tables in linux/arch/i386/head.S

I search the internet again and again but fail to find any answer
so far, maybe you gurus can help me out, here it goes:

48         cld
49         movl $(__KERNEL_DS),%eax
50         movl %eax,%ds
51         movl %eax,%es
52         movl %eax,%fs
53         movl %eax,%gs
81 /*
82  * Initialize page tables
83  */
84         movl $pg0-__PAGE_OFFSET,%edi /* initialize page tables */
85         movl $007,%eax  /* "007" doesn't mean with right to kill, but
86                                    PRESENT+RW+USER */
87 2:      stosl
88         add $0x1000,%eax
89         cmp $empty_zero_page-__PAGE_OFFSET,%edi
90         jne 2b
   
I remove the SMP code.  According the setup.S, gdt_table is setup as
gdt_table:		
			#.quad 0x0000000000000000;	// null
			#.quad 0x0000000000000000;	// not used
			#.quad 0x00cf9a000000ffff;	// 0x10 kernel 4GB code at 0x00000000
			#.quad 0x00cf92000000ffff;	// 0x18 kernel 4GB data at 0x00000000

1) So, what's in %eax after line 49 ?  0x0 ?
2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?

Thanks,

Jason

Re: paging question

[Richard B. Johnson]

On Wed, 20 Feb 2002, Jason Yan wrote:

> Hi,
> 
> I have a question about the code to enable  to initialize page 
> tables in linux/arch/i386/head.S
> 
> I search the internet again and again but fail to find any answer
> so far, maybe you gurus can help me out, here it goes:
> 
> 48         cld
> 49         movl $(__KERNEL_DS),%eax
> 50         movl %eax,%ds
> 51         movl %eax,%es
> 52         movl %eax,%fs
> 53         movl %eax,%gs
> 81 /*
> 82  * Initialize page tables
> 83  */
> 84         movl $pg0-__PAGE_OFFSET,%edi /* initialize page tables */
> 85         movl $007,%eax  /* "007" doesn't mean with right to kill, but
> 86                                    PRESENT+RW+USER */
> 87 2:      stosl
> 88         add $0x1000,%eax
> 89         cmp $empty_zero_page-__PAGE_OFFSET,%edi
> 90         jne 2b
>    
> I remove the SMP code.  According the setup.S, gdt_table is setup as
> gdt_table:		
> 			#.quad 0x0000000000000000;	// null
> 			#.quad 0x0000000000000000;	// not used
> 			#.quad 0x00cf9a000000ffff;	// 0x10 kernel 4GB code at 0x00000000
> 			#.quad 0x00cf92000000ffff;	// 0x18 kernel 4GB data at 0x00000000
> 
> 1) So, what's in %eax after line 49 ?  0x0 ?
> 2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?
> 
> Thanks,
> 
> Jason

The discriptor for the segments are in the gdt_table. They each consist
of a start a length a type and a granularity. They are not NUMBERS. They
are parameters that the CPU understands. They define the linear address
space, to be accessed, and the protections for that address space. They
have nothing to do with paging.

The GDT element, so defined, is called a selector or a segment. Each
of the elements is a guad-word. The offset, neglecting the first
element is 0x18 for the data-segment. Therefore KERNEL_DS is really
0x18. This is loaded into a register (%eax), and then loaded into
the relevant segments. You can't load a segment register directly
with a value. You have to transfer the contents of another register
or memory into that segment.

Paging, is different. First somewhere there has to be a 1:1 translation
of physical to virtual addresses so that the kernel can always access
page-tables. These page-tables are some place where there is such a
correspondance. Then, every page that exists, anywhere, has a page-table
entry. These PTEs can be marked as present or not to generate traps
(page faults). The PTEs allow any physical chunk of memory (page) from
anywhere, to be mapped into a processes virtual address space. The mapping
creates, in addition to a 0->ffffffff virtual address range for every
process, a boundary, called PAGE_OFFSET. To user-mode programmers, it
doesn't exist. Only in kernel-mode does it mean anything and, in fact,
macros have been defined to hide its meaning. In the Intel machines, the
kernel uses virtual addresses just like processes do. However, above the
address-range where there is a 1:1 virtual:physical address translation,
the virtual address comes out to be virtual = physical | PAGE_OFFSET. You
should never use this knowledge in any module or you will find that
it will break (and you get to keep the pieces) when new kernel versions
are available.

> 2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?
If my hex-math is correct,  bff00000.  But don't use numbers. Let the
compiler/assembler do the math between labels. That way, you can add stuff
and it still works.



Cheers,
Dick Johnson

Penguin : Linux version 2.4.1 on an i686 machine (797.90 BogoMips).

        111,111,111 * 111,111,111 = 12,345,678,987,654,321

Re: paging question

[Cesar Suga]

On Wed, 20 Feb 2002, Jason Yan wrote:

> I have a question about the code to enable  to initialize page
> tables in linux/arch/i386/head.S

> I search the internet again and again but fail to find any answer
> so far, maybe you gurus can help me out, here it goes:

> 48         cld
> 49         movl $(__KERNEL_DS),%eax
> 50         movl %eax,%ds
> 51         movl %eax,%es
> 52         movl %eax,%fs
> 53         movl %eax,%gs
> 81 /*
> 82  * Initialize page tables
> 83  */
> 84         movl $pg0-__PAGE_OFFSET,%edi /* initialize page tables */
> 85         movl $007,%eax  /* "007" doesn't mean with right to kill, but
> 86                                    PRESENT+RW+USER */
> 87 2:      stosl
> 88         add $0x1000,%eax
> 89         cmp $empty_zero_page-__PAGE_OFFSET,%edi
> 90         jne 2b
>
> I remove the SMP code.  According the setup.S, gdt_table is setup as
> gdt_table:
> 			#.quad 0x0000000000000000;	// null
> 			#.quad 0x0000000000000000;	// not used
> 			#.quad 0x00cf9a000000ffff;	// 0x10 kernel 4GB code at 0x00000000
> 			#.quad 0x00cf92000000ffff;	// 0x18 kernel 4GB data at 0x00000000
>
> 1) So, what's in %eax after line 49 ?  0x0 ?

	As in asm/segment.h (included on arch/i386/kernel/head.S): (i386)

#ifndef _ASM_SEGMENT_H
#define _ASM_SEGMENT_H

#define __KERNEL_CS     0x10
#define __KERNEL_DS     0x18

	uaccess.h specifies another value, but that's KERNEL_DS for user
space memory.

> 2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?

	Yes, __PAGE_OFFSET is 0xC0000000. I am not sure, but I think it
initializes the page tables to 8MB here, and the final page tables depend
on the memory size (better seeing pgtable.h)...

	Regards,
	Cesar Suga <sartre@linuxbr.com>

Re: paging question

[Gianni Tedesco]

On Wed, 2002-02-20 at 21:26, Jason Yan wrote:
> Hi,
> 
> I have a question about the code to enable  to initialize page 
> tables in linux/arch/i386/head.S
> 
> I search the internet again and again but fail to find any answer
> so far, maybe you gurus can help me out, here it goes:
> 
> 48         cld
> 49         movl $(__KERNEL_DS),%eax
> 50         movl %eax,%ds
> 51         movl %eax,%es
> 52         movl %eax,%fs
> 53         movl %eax,%gs
> 81 /*
> 82  * Initialize page tables
> 83  */
> 84         movl $pg0-__PAGE_OFFSET,%edi /* initialize page tables */
> 85         movl $007,%eax  /* "007" doesn't mean with right to kill, but
> 86                                    PRESENT+RW+USER */
> 87 2:      stosl
> 88         add $0x1000,%eax
> 89         cmp $empty_zero_page-__PAGE_OFFSET,%edi
> 90         jne 2b
>    
> I remove the SMP code.  According the setup.S, gdt_table is setup as
> gdt_table:		
> 			#.quad 0x0000000000000000;	// null
> 			#.quad 0x0000000000000000;	// not used
> 			#.quad 0x00cf9a000000ffff;	// 0x10 kernel 4GB code at 0x00000000
> 			#.quad 0x00cf92000000ffff;	// 0x18 kernel 4GB data at 0x00000000
> 
> 1) So, what's in %eax after line 49 ?  0x0 ?

0x18, its the index of the 4th item in GDT. Lines 49-53 set the segment
registers to use the settings in gdt[3];

> 2) Isn't __PAGE_OFFSET 0xC0000000 ? what's the result of $pg0-__PAGE_OFFSET ?

the physical address of $pg0, the kernel is linked to PAGE_OFFSET so it
thinks the address of $pg0 is PAGE_OFFSET+something. Suptracting
PAGE_OFFSET is the simple way to obtain the physical address.

-- 
// Gianni Tedesco <gianni@ecsc.co.uk>
80% of all email is a figment of procmails imagination.