newbie question about 2.6 ia64 linux-gate etc

newbie question about 2.6 ia64 linux-gate etc

[Raj Patil]

Hello,

I am looking for some basic info on how the linux-gate.so is used and how it
gets mapped to the process address space in 2.6 ia64 and how all the system 
call
function names gets included here? (I have access to SuSE 9.0, based on 
2.6.5-7.97)

Can some please point/direct  me to some doc/notes, if it exists or write 
few lines to
give some basic information information about these?

Thank you very much and really appreciate info.

Regards,
Raj

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Re: newbie question about 2.6 ia64 linux-gate etc

[David Mosberger]

>>>>> On Mon, 25 Oct 2004 10:57:25 -0400, "Raj Patil" <rpatil0299@hotmail.com> said:

  Raj> Hello, I am looking for some basic info on how the
  Raj> linux-gate.so is used and how it gets mapped to the process
  Raj> address space in 2.6 ia64 and how all the system call function
  Raj> names gets included here? (I have access to SuSE 9.0, based on
  Raj> 2.6.5-7.97)

  Raj> Can some please point/direct me to some doc/notes, if it exists
  Raj> or write few lines to give some basic information information
  Raj> about these?

  Raj> Thank you very much and really appreciate info.

See the last section "Using fast system calls" of

	Documentation/ia64/fsys.txt

I'll append it below for convenience.

BTW: The vDSO approach isn't unique to ia64.  It was invented for x86
and other platforms use it as well.

	--david


----
* Using fast system calls

To use fast system calls, userspace applications need simply call
__kernel_syscall_via_epc().  For example

-- example fgettimeofday() call --
-- fgettimeofday.S --

#include <asm/asmmacro.h>

GLOBAL_ENTRY(fgettimeofday)
.prologue
.save ar.pfs, r11
mov r11 = ar.pfs
.body 

mov r2 = 0xa000000000020660;;  // gate address 
			       // found by inspection of System.map for the 
			       // __kernel_syscall_via_epc() function.  See
			       // below for how to do this for real.

mov b7 = r2
mov r15 = 1087		       // gettimeofday syscall
;;
br.call.sptk.many b6 = b7
;;

.restore sp

mov ar.pfs = r11
br.ret.sptk.many rp;;	      // return to caller
END(fgettimeofday)

-- end fgettimeofday.S --

In reality, getting the gate address is accomplished by two extra
values passed via the ELF auxiliary vector (include/asm-ia64/elf.h)

 o AT_SYSINFO : is the address of __kernel_syscall_via_epc()
 o AT_SYSINFO_EHDR : is the address of the kernel gate ELF DSO

The ELF DSO is a pre-linked library that is mapped in by the kernel at
the gate page.  It is a proper ELF shared object so, with a dynamic
loader that recognises the library, you should be able to make calls to
the exported functions within it as with any other shared library.
AT_SYSINFO points into the kernel DSO at the
__kernel_syscall_via_epc() function for historical reasons (it was
used before the kernel DSO) and as a convenience.