Messages in this thread | | | Date | Fri, 20 Dec 2002 23:38:25 +0000 | From | Jamie Lokier <> | Subject | Re: Intel P6 vs P7 system call performance |
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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/
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