linux-kernel - Re: [PATCHv2 2/4] x86_64: embrace KERNEL_STACK

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Message-Id: <1421857754.173957.216801721.380D3A16@webmail.messagingengine.com>
Date:	Wed, 21 Jan 2015 17:29:14 +0100
From:	Alexander van Heukelum <heukelum@...tmail.fm>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>, Borislav Petkov <bp@...e.de>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCHv2 2/4] x86_64: embrace KERNEL_STACK_OFFSET

On Wed, Jan 21, 2015, at 14:44, Denys Vlasenko wrote:
> On Sun, Jan 18, 2015 at 12:45 PM, Alexander van Heukelum
> <heukelum@...tmail.fm> wrote:
> > KERNEL_STACK_OFFSET is the offset from the top of the kernel stack
> > page to the value of the kernel_stack percpu variable. This patch
> > changes KERNEL_STACK_OFFSET to configure a reserved space of 16
> > bytes above the user ptregs frame. KERNEL_STACK_OFFSET must be
> > set to a multiple of 16 bytes due to the automatic stack alignment
> > of interrupts, traps, and exceptions on x86_64.
> 
> I propose to set kernel_stack percpu variable to point
> to the top of kernel stack (obvious, isn't it?)
> and eliminate KERNEL_STACK_OFFSET altogether.

By "top of kernel stack", do you mean the page boundary or the
top of struct pt_regs on the kernel stack? (is it really that obvious?)
I think Borislav did the latter for x86_64 in his patchset.

Eliminating KERNEL_STACK_OFFSET was what I did in my first
attempt, and I broke i386 by not thinking through what x86 common
code was really doing :).
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