Task Switching using ( TSS ) ?

[amirsadig]

I implement now task switching without "setjmp" , "longjmp" functions. I would like to use TSS. I have successfully define a TSS Structure and
and load the initial value for the first Task "init" in the TR Register and I have not receive any error. that mean the CPU has accepted my TSS.

now I have define a second task, but I don't know how to make far call or jmp to that task " tss_sekector:tss->eip" with inline assembly in gcc.

I save the value of "tr" in the task_struct and also the TSS.
in gcc I have done that :
__asm__(      
    " ljmp %0, %1 \n"   
    ::"m"(current_task->tr), "m"(current_task->tss->eip)
    )
then I receive this compiler error:
"too many memory references for `ljmp"

and when I use for "c" for the second operand I receive this :
"suffix or operands invalid for `ljmp'".

:-[

[Pype.Clicker]

i think you should take back your opcode list and check what "ljmp" expects as arguments. afaik, you have the choice between JMP <constant segment>:<constant offset> or JMP [<memory>] where <memory> is the address of a far pointer (segment & offset where you'd like to jump to).

[amirsadig]

the syntax for ljmp in AT&T syntax is :

ljmp segment, offset

example:

ljmp $0x18, $0x10080

[Pype.Clicker]

and these are constants. That's why your compiler complains when you give him 2 "m" instead

and he doesn't like "c" (which is %ecx, iirc) aswell.

[amirsadig]

then have a solution ? I have search the internet, but no solutions found.

[amirsadig]

could somebody declare me, why this code work to switch to the new TSS :

__asm__(      
    " ljmp %0 \n"      
    ::"m"(*(((char *)?t_task->tss->tr)-4))
    );

I have found something like this in the first linux version, but there is no declaration.

change the address of current_task->tss->tr to (char *) and then minus 4, then took the value at that address. is right what that code does?

[Pype.Clicker]

__asm__(
" ljmp %0 \n"
::"m"(*(((char *)?t_task->tss->tr)-4))
);

I have found something like this in the first linux version, but there is no declaration.
is right what that code does?

You should try to find out what that 'TR' field is about. afaik, there is no TR field in a TSS. what i suppose is that the TSS structure has an undocumented field (which has no name) 4 bytes before the "tr" field (maybe this is "BACK" link ?), so they took the address of tss->tr, cast it as a char* (so that pointer arithmetics now have a 1-byte increment) and pick the value at that place to be a far-pointer to the next task.

then a "ljmp [tr-address]" is performed.

[amirsadig]

my TSS structure has all TSS information plus the pointer to 33 byte (IO Bitmap) and the tr.
therefor the above code the point to the TOP of the TSS ( bitmap). This what I dont't understand, how the CPU then know the TSS Selector?
--------------
struct tss_struct {
   unsigned short   back_link,__blh;
   unsigned long   esp0;
   unsigned short   ss0,__ss0h;
   unsigned long   esp1;
   unsigned short   ss1,__ss1h;
   unsigned long   esp2;
   unsigned short   ss2,__ss2h;
   unsigned long   cr3;
   unsigned long   eip;
   unsigned long   eflags;
   unsigned long   eax,ecx,edx,ebx;
   unsigned long   esp;
   unsigned long   ebp;
   unsigned long   esi;
   unsigned long   edi;
   unsigned short   es, __esh;
   unsigned short   cs, __csh;
   unsigned short   ss, __ssh;
   unsigned short   ds, __dsh;
   unsigned short   fs, __fsh;
   unsigned short   gs, __gsh;
   unsigned short   ldt, __ldth;
   unsigned short   trace, bitmap;
   unsigned long   io_bitmap[IO_BITMAP_SIZE+1];
   unsigned long   tr; // save the TSS selector (the Kernel change it during task init..)
   unsigned long   cr2, trap_no, error_code;
};

[Pype.Clicker]

okay. This is because ljmp expects a "far pointer" :

Code: Select all

fp: db 78,56,34,12,CD,AB
jmp far [fp]

is equivalent to

Code: Select all

jmp far ABCD:12345678

In our case, the segment is a task gate, which means the offset will be just dropped. So what Linus&co made is to let the offset be some garbage (in the IO map) and have the segment being TR.
hence the "-4" which goes backward to have iomap[N-3..N] as the offset part.

[amirsadig]

hi, Thank you very much for help. thanks again.