Re: Intel P6 vs P7 system call performance

[Linus Torvalds]

On Tue, 17 Dec 2002, Linus Torvalds wrote:
>
> Hmm.. Which system calls have all six parameters? I'll have to see if I
> can find any way to make those use the new interface too.

The only ones I found from a quick grep are
 - sys_recvfrom
 - sys_sendto
 - sys_mmap2()
 - sys_ipc()

and none of them are of a kind where the system call entry itself is the
biggest performance issue (and sys_ipc() is deprecated anyway), so it's
probably acceptable to just use the old interface for them.

One other alternative is to change the calling convention for the
new-style system call, and not have arguments in registers at all. We
could make the interface something like

 - %eax contains system call number
 - %edx contains pointer to argument block
 - call *syscallptr	// trashes all registers

and then the old "compatibility" function would be something like

	movl 0(%edx),%ebx
	movl 4(%edx),%ecx
	movl 12(%edx),%esi
	movl 16(%edx),%edi
	movl 20(%edx),%ebp
	movl 8(%edx),%edx
	int $0x80
	ret

while the "sysenter" interface would do the loads from kernel space.

That would make some things easier, but the problem with this approach is
that if you have a single-argument system call, and you just pass in the
stack pointer offset in %edx directly, then the system call stubs will
always load 6 arguments, and if we're just at the end of the stack it
won't actually _work_. So part of the calling convention would have to be
the guarantee that there is stack-space available (should always be true
in practice, of course).

			Linus