Re: Intel P6 vs P7 system call performance

[dada1]

> For the libc DSO I had to play some dirty tricks.  The x86 CPU has no
> absolute call.  The variant with an immediate parameter is a relative
> jump.  Only when jumping through a register or memory location is it
> possible to jump to an absolute address.  To be clear, if I have
>
>     call 0xfffff000
>
> in a DSO which is loaded at address 0x80000000 the jumps ends at
> 0x7fffffff.  The problem is that the static linker doesn't know the load
> address.  We could of course have the dynamic linker fix up the
> addresses but this is plain stupid.  It would mean fixing up a lot of
> places and making of those pages covered non-sharable.
>

You could have only one routine that would need a relocation / patch at
dynamic linking stage :

absolute_syscall:
    jmp  0xfffff000

Then all syscalls routine could use :

getpid:
    ...
    call absolute_syscall
    ...
instead of "call 0xfffff000"

If the kernel doesnt support the 0xfffff000 page, you could patch
absolute_syscall (if it resides in .data section) with :
    absolute_syscall:
            int 0x80
            ret
(3 bytes instead of 5 bytes)

See you