Hi:
Thanks for steven. but I think you don't understand real intention
of my question.
First, please allow me make a bit explain about intel architecture.
If it include
any error, please tell me. 3ks.
On i386, that all addresses can be used directly in program, is
called "logical address",
logical address include a pointer(16bit) to segment descriptor table and
one offset(32bit).
the logical address must be pass segment translation. this process use
segment descriptor in
GDT or LDT (Global/Local segment Descriptor Table). Each segment
descriptor includes
one base address of the segment, then, the offset in "logical address"
and this base address
will combine "linear address", if we disable paging, this linear address
just is physcial address.
however, if we enable great paging, it still need do paging translate to
physical address yet.
In paging process, CR3 register take one important role, it include page
table directory base address.
On i386 Linux, segment tranlstion just is dummy process, all logical
address is translated to same
address. in other words, all segments (KERNEL_CS, KERNEL_DS, USER_DS,
USER_CS) have zero value base
address.
And we must note, in above words, the terms "logical address" ,
"linear address" are came from intel
architecture manual, they are not completely equal with linux kernel
terms. but both "linear address" are
the most like.
The paging at i386 architecture must use CR3 register, as we known,
at least. So in linux kernel, if it
is going to map its "linear address" to low end physical address, it
also need use CR3 register. but it can
not use CR3 directly ,in switch_mm() function at least, this function
will change CR3 register value to switch
user task memory address space.
I known kernel often do not setup page table for itself, except some
special cases, for example, vmalloc.
This feature say kernel use page table(and CR3) in reverse.
I want to know how kernel translate itself address , especially, How
code after kernel change CR3 register
work? It use CR3, or no? As steven said, I am confused here really.
It is like kernel have many secrets I don't know. these secrets
drive me study it.
3ks in advanced.
liyu/NOW~
Steven Rostedt wrote:
Hi Liyu,
I'm not that strong in the Intel world, but after all the fancy
registers, intel is not much different than say PPC. So I'll keep this
more of a generic platform discussion, and only talk about physical and
virtual address space.
The kernel is usually mapped down to the lower end of memory, which in
most platforms starts at physical address zero (I've worked with
platforms that don't do this, but that's offtopic). Then the kernel maps
this physical location to some upper address (with intel it's usually
0xc0000000).
All the user space addresses are mapped below this kernel address. Now
the magic here, and it probably confuses you, is that all
tasks/processes have the kernel address mapped to the same location. So
on context switches, the kernel is still in the same location in virtual
address. It's just that when the CPU is in user mode, the kernel
address is protected from being read or written to. So if a user space
process tries to write or read from it (like *(char*)(0xc0000000) = 1;)
it will get a page fault. But when the CPU switches to kernel mode
(through a system call or interrupt), it has full access to this area.
So you have this mapping:
Physical Virtual
0x00000000 ------------ -------------
| kernel | --+ | user |
| | | | |
+----------+ | | |
| general | | | |
| memory | | | |
. . | . .
. . | . .
| end of | | | |
| memory | | | |
+----------+ | | |
| | +---> +------------+ 0xc0000000
| | | kernel |
. . . .
. . . .
Usually all of memory is mapped to the address 0xc0000000. But this
becomes a problem when you have a gig or more of RAM. Since you run out
of virtual space to map there. And you still need room to map device
memory as well (you can't have user space conflicting with devices). So
if you have a lot of RAM, you need to turn on highmem support, which
then plays around to get memory above a certain point. But that's
another discussion.
So to access physical memory from the kernel, you can use __pa and back
with __va. These are used to communicate with devices usually, which
are also mapped to some location. But these only work when the mapping
is direct as show above. When highmem support is on, you can't get to
memory that is not mapped in. But there's no need to use __pa or __va to
get to memory just for itself. Usually they are used when dealing with
devices that have DMA or some other need to find a physical address. If
a device needs to write to memory (usually only knowing about the
physical location of the memory) you get that memory with GFP_DMA flag,
which guarantees that you will get memory that is mapped directly.
-- Steve
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