linux-kernel - Re: [PATCH 2/4] x86: get rid of KERNEL_STACK

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Date:	Tue, 24 Feb 2015 14:30:04 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] x86: get rid of KERNEL_STACK_OFFSET

On Tue, 24 Feb 2015 19:51:33 +0100
Denys Vlasenko <dvlasenk@...hat.com> wrote:

> PER_CPU_VAR(kernel_stack) was set up in a way where it points
> five stack slots below the top of stack.
> 
> Presumably, it was done to avoid one "sub $5*8,%rsp"
> in syscall/sysenter code paths, where iret frame needs to be
> created by hand.
> 
> Ironically, none of them benefit from this optimization,
> since all of them need to allocate additional data on stack
> (struct pt_regs), so they still have to perform subtraction.
> And ia32_sysenter_target even needs to *undo* this optimization:
> it constructs iret stack with pushes instead of movs,
> so it needs to start right at the top.
> 
> This patch eliminates KERNEL_STACK_OFFSET.
> PER_CPU_VAR(kernel_stack) now points directly to top of stack.
> pt_regs allocations are adjusted to allocate iret frame as well.
> 

I always thought the KERNEL_STACK_OFFSET wasn't an optimization, but a
buffer from the real top of stack, in case we had any off by one bugs,
it wouldn't crash the system.

-- Steve
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