linux-kernel - Re: [PATCHv4 5/6] Allow setting O

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Date:	Mon, 26 Nov 2007 18:38:17 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ulrich Drepper <drepper@...hat.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, tglx@...utronix.de
Subject: Re: [PATCHv4 5/6] Allow setting O_NONBLOCK flag for new sockets

Linus Torvalds wrote:
> 
>> The 6-word limit is a red herring.  There is at least two ways to deal with it
>> (and this doesn't mean wiping the legacy stuff we already have):
>>
>> - Let each architecture pick a calling convention and redefine the
>> architecture-independent bits to take an arbitrary number of arguments.  This
>> is a one-time panarchitectural change.
> 
> Not applicable on x86-32.
> 
> The six-word limit is effectively a hardware limit there. Once it goes 
> past that limit, one of the words needs to be a pointer to extended 
> information that is fundamentally slower to access. Happily, only very 
> rare system calls do that (and none of them are of the simple variety 
> where we see a few cycles easily).
> 
> On other architectures, we could more easily just use more registers. But 
> x86-32 is still a big part (bulk) of what matters for most people.
> 

Well, x86-32 and x86-64 are surprisingly similar here, for very 
different reasons (x86-64 is because there are only seven clobbered 
registers that aren't destroyed by the syscall instruction itself.)

However, on both of these we could make the user-space side cheaper, by 
making sure that we don't have to do additional copies in user space. 
For both these architectures, anything more than 3 parameters (i386) or 
6 parameters (x86-64) will be already in memory on the stack, so if we 
can use that image as-is then we at least save the intra-user-space copy 
that goes along with it.

x86-64 requires some minor thought, since the obvious way of doing it 
(using arg register 6 to push in a pointer) would end up with a 
discontiguous frame.  One can do it with something like this, although 
it's not clear to me it is a win at all (the more obvious sequence using 
XCHG isn't usable since XCHG locks unconditionally):

	pop	%r10			# Return address
	push	%r9			# Argument 6
	movq	%rsp, %r11
	push	%r10
	movq	%rcx, %r10
	syscall
	cmpq	$-4095, %rax
	jae	...
	pop	%r10
	pop	%r9
	push	%r10
	retq

The number of registers do vary, obviously, with s390 being the smallest 
number (5).

> Immediately when you do anything but registers, it is much *much* more 
> costly. The "get_user()" and "copy_from_user()" stuff is not exactly slow, 
> but it's quite noticeable overhead for simple system calls. It gets worse 
> if this all is described by some indirect table setup.

True, of course, although we're talking here about different ways to 
pull arguments out of userspace memory; *definitely* agreed with that we 
don't want to have any additional indirection necessary.

	-hpa
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