Re: Intel P6 vs P7 system call performance

[H. Peter Anvin]

Jamie Lokier wrote:
> 
> 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

... or just...

getpid:
	movl $__NR_getpid, %eax
	jmp *%gs:0x18

This doesn't mess up the call/return stack, even, because the ret in the
stub matches the call to getpid.

	-hpa