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

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Date:	Mon, 23 Mar 2015 18:51:11 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 03/23/2015 06:45 PM, Steven Rostedt wrote:
> On Mon, 23 Mar 2015 18:38:09 +0100
> Denys Vlasenko <dvlasenk@...hat.com> wrote:
> 
>>> Can we do a:
>>>
>>>  #define cpu_current_top_of_stack (cpu_tss + TSS_sp0)
>>
>> We already do something similar:
>>
>> static inline unsigned long current_top_of_stack(void)
>> {
>> #ifdef CONFIG_X86_64
>>         return this_cpu_read_stable(cpu_tss.x86_tss.sp0);
>> #else
>>         /* sp0 on x86_32 is special in and around vm86 mode. */
>>         return this_cpu_read_stable(cpu_current_top_of_stack);
>> #endif
>> }
> 
> So can we then have:
> 
> #ifdef __ASSEMBLY__
> # define cpu_current_top_of_stack (cpu_tss + TSS_sp0)
> #else
> # define cpu_current_top_of_stack (cpu_tss.x86_tss.sp0)
> #endif
> 
> And get rid of that if statement in the static inline?

I prefer less macro indirection in assembly code,
but I won't object too strongly if this would be done.

It's up to other x86 maintainers to agree - I'm touching
the code recently changed by Andy, he may see the way forward
somewhat differently.

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