linux-kernel - Re: [PATCH 2/3] x86: Pass in pt_regs pointer for syscalls that need it

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Message-ID: <49932E51.8040009@zytor.com>
Date:	Wed, 11 Feb 2009 12:00:17 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Brian Gerst <brgerst@...il.com>
CC:	"H. Peter Anvin" <hpa@...nel.org>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] x86: Pass in pt_regs pointer for syscalls that need
 	it

Brian Gerst wrote:
> 
> IMHO, copying the 4th-6th args to a new stack frame is the only way to
> guarantee that gcc won't trash any part of pt_regs.  The question is
> whether to do it unconditionally, or try to be clever and only copy
> them for the syscalls that actually need them.
> 

My guess is that the conditionalization would actually cost more than 
doing it unconditionally.  We're talking a small fraction of a cache 
line, and a set of stores to RAM, which can be buffered.

It's in many ways easier than reorganizing the struct pt_regs.  I'm just 
hypersensitive to adding system call overhead in any way.

	-hpa

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