Re: Intel P6 vs P7 system call performance

From: Jamie Lokier (lk@tantalophile.demon.co.uk)
Date: Fri Dec 20 2002 - 18:38:25 EST


Linus Torvalds wrote:
> And if the caller cannot depend on registers being saved, the caller may
> actually end up being more complicated. For example, with the current
> setup, you can have
>
> getpid():
> movl $__NR_getpid,%eax
> jmp *%gs:0x18
>
> but if system calls clobber registers, the caller needs to be
>
> [long code snippet]
>
> and notice how the _real_ code sequence actually got much _worse_ from the
> fact that you tried to save time by not saving registers.

No, your "real" code sequence is wrong.

%ebx/%edi/%esi are preserved across sysenter/sysexit, whereas
%ecx/%edx are call-clobbered registers in the i386 function call ABI.

This is not a coincidence.

So, getpid looks like this with the _smaller_ vsyscall code:

         getpid():
                 movl $__NR_getpid,%eax
                 call *%gs:0x18
                 ret

Intel didn't choose %ecx/%edx as the sysexit registers by accident.
They were chosen for exactly this reason.

By the way, the same applies to AMD's syscall/sysret, which clobbers %ecx.

What I'm suggesting is that we should say that "call 0xffffe000"
clobbers only the registers (%eax/%ecx/%edx) that _normal_ function
calls clobber on i386, and preserves the call-saved registers.

This keeps the size of system call stubs in libc to the minimum.
Think about it.

-- Jamie
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