linux-kernel - Re: [PATCH 1/2] x86: stop using PER_CPU_VAR(kernel

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Date:	Mon, 23 Mar 2015 13:28:47 -0400
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 1/2] x86: stop using PER_CPU_VAR(kernel_stack)

On Mon, 23 Mar 2015 18:10:01 +0100
Denys Vlasenko <dvlasenk@...hat.com> wrote:

> On 03/23/2015 03:18 PM, Steven Rostedt wrote:
> > On Sat, 21 Mar 2015 21:44:37 +0100
> > Denys Vlasenko <dvlasenk@...hat.com> wrote:
> > 
> >> Instead of PER_CPU_VAR(kernel_stack), 64-bit code
> >> can use PER_CPU_VAR(cpu_tss + TSS_sp0).
> > 
> > The change log here is lacking an answer to "why". It only states what
> > it does. What's wrong with using kernel_stack? The change log should
> > explicitly state that. I have no idea why this patch is needed.
> 
> Sorry. The reason is:
> 
> We want to get rid of kernel_stack, since it is redundant:
> in 64-bits, PER_CPU_VAR(cpu_tss + TSS_sp0) can be used instead,
> in 32-bits, PER_CPU_VAR(cpu_current_top_of_stack) can be used instead.

Can we do a:

 #define cpu_current_top_of_stack (cpu_tss + TSS_sp0)

in 64-bit, and make it consistent with i386.

"cpu_tss + TSS_sp0" is rather meaningless. "kernel_stack" or
"top_of_stack" is at least informative.

> 
> Patch 2/2 in the same series removes kernel_stack.

Understood, but each commit's change log should be able to stand on its
own.

-- Steve


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