linux-kernel - Re: [PATCHv3 0/4] sys

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Date:	Sun, 18 Nov 2007 11:37:40 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ulrich Drepper <drepper@...hat.com>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, tglx@...utronix.de, torvalds@...ux-foundation.org
Subject: Re: [PATCHv3 0/4] sys_indirect system call

Ulrich Drepper wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> H. Peter Anvin wrote:
>> It seems to me that we could accomplish the same thing by passing the
>> number of parameters in the upper bits of the system call number
>> register (%eax in the case of x86.)
> 
> This isn't really a generic solution.  The number of parameters is
> limited to six.  There are syscalls with six parameters already.  There
> are many more with five which could only handle one more parameter.
> 
> Also, is it really simpler?  You'd need to have another table which
> contains the default number of parameters a system call takes so that
> you can fill in the default value of zero.  This extra memory access has
> to be performed for every system call.
> 
> I think it is unlikely that this approach is faster.  To the contrary,
> I'd guess.
> 
> I don't have much invested into this but it seems the sys_indirect
> approach is so much simpler.  Overhead is only paid if you really need
> it which is rarely the case.  Plus, you might have heard Linus and Zack
> talk about syslets again.  Starting syslets can be done using the same
> interface, I guess.
> 

What bothers me about the sys_indirect approach is that it will get 
increasingly expensive as time goes on, and in doing so it does a 
user-space memory reference, which are extra expensive.  The extra table 
can be colocated with the main table (a structure, in effect) so they'll 
share the same cache line.

	-hpa
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