lkml.org 
[lkml]   [2002]   [Dec]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Intel P6 vs P7 system call performance
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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:31    [W:0.103 / U:0.576 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site